Improve validation output in elyra-pipeline CLI

[ptitzler]

What changes were proposed in this pull request?

This PR:

• improves visualization of pipeline validation results
• simplifies existing code to reduce maintenance efforts
• updates pipeline validation messages for clarity (wording, punctuation, etc)
• adds incorrect property value to validation issue payload to aid with troubleshooting
• fixes validation bugs

The proposed output introduces a new generic message display "pattern", comprising

• [validation-issue-severity] - always displayed
• [node-label] - displayed if applicable and available
• [property-name] - displayed if applicable and available
• - <validation-message-text> - always displayed

Messages are sorted by severity (error > warning > ...), enabling the user to quickly locate issues that cause the operation to be aborted.

For clarity each component could be prefixed, but that would result in a lot more "static" output.

Example output without prefixes:

$ elyra-pipeline submit a-circle.pipeline --runtime-config my_airflow_test_instance

────────────────────────────────────────────────────────────────
 Elyra Pipeline Submission
────────────────────────────────────────────────────────────────

Validating pipeline...

[Error][i depend on you][runtime_image] - Required property value is missing. 
[Error][you depend on me][env_vars] - Property has an improperly formatted env variable key value pair. The current property value is 'what up'.
[Error][hello.ipynb][runtime_image] - Required property value is missing. 
[Error][labeled-notebook-nod][env_vars] - Property has an improperly formatted env variable key value pair. The current property value is 'humpty goes dumpty'.
[Error] - The pipeline contains a circular dependency between nodes. Nodes: 'you depend on me', 'i depend on you'
[Warning][i depend on you][label] - The node label contains characters that may be replaced by the runtime service. Node labels should start with lower case alphanumeric and contain only lower case alphanumeric, underscores, dots, and dashes. The current property value is 'i depend on you'.
[Warning][you depend on me][label] - The node label contains characters that may be replaced by the runtime service. Node labels should start with lower case alphanumeric and contain only lower case alphanumeric, underscores, dots, and dashes. The current property value is 'you depend on me'.
[Warning][hello.ipynb] - Node is not connected to any other node. 
[Warning][labeled-notebook-nod] - Node is not connected to any other node. 

Example output with prefixes:

$ elyra-pipeline submit a-circle.pipeline --runtime-config my_airflow_test_instance
────────────────────────────────────────────────────────────────
 Elyra Pipeline Submission
────────────────────────────────────────────────────────────────
Validating pipeline...

[Error][node: i depend on you][property:runtime_image] - Required property value is missing. 
[Error][node: you depend on me][property: env_vars] - Property has an improperly formatted env variable key value pair. The current property value is 'what up'.

How was this pull request tested?

Reviewed output for the following scenarios

• validation output (no errors or warnings) run command

• validation output (errors but no warnings) run command

• validation output (warnings only) run command

• validation output (no errors or warnings) submit command

• validation output (errors but no warnings) submit command

• validation output (warnings only) submit command

• verify that validation changes don't break VPE output

Fixes #1999

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[elyra-bot]

Thanks for making a pull request to Elyra!

To try out this branch on binder, follow this link:

[ptitzler]

There is a known issue related to custom component validation that is not handled by this PR. For example,

$ elyra-pipeline submit custom-not-found.pipeline --runtime-config my_kubeflow_test_instance

────────────────────────────────────────────────────────────────
 Elyra Pipeline Submission
────────────────────────────────────────────────────────────────

Validating pipeline...
Traceback (most recent call last):
  File "/opt/anaconda3/envs/pr2070/bin/elyra-pipeline", line 8, in <module>
    sys.exit(pipeline())
  File "/opt/anaconda3/envs/pr2070/lib/python3.7/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/opt/anaconda3/envs/pr2070/lib/python3.7/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/opt/anaconda3/envs/pr2070/lib/python3.7/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/opt/anaconda3/envs/pr2070/lib/python3.7/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/opt/anaconda3/envs/pr2070/lib/python3.7/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/opt/anaconda3/envs/pr2070/lib/python3.7/site-packages/elyra/cli/pipeline_app.py", line 278, in submit
    _validate_pipeline_definition(pipeline_definition)
  File "/opt/anaconda3/envs/pr2070/lib/python3.7/site-packages/elyra/cli/pipeline_app.py", line 198, in _validate_pipeline_definition
    PipelineValidationManager.instance().validate(pipeline=pipeline_definition))
  File "/opt/anaconda3/envs/pr2070/lib/python3.7/asyncio/base_events.py", line 587, in run_until_complete
    return future.result()
  File "/opt/anaconda3/envs/pr2070/lib/python3.7/site-packages/elyra/pipeline/validate.py", line 123, in validate
    pipeline_execution=pipeline_execution)
  File "/opt/anaconda3/envs/pr2070/lib/python3.7/site-packages/elyra/pipeline/validate.py", line 300, in _validate_node_properties
    property_dict['current_parameters'].keys()))
KeyError: 'current_parameters'