[AIRFLOW-2608] Implements/Standardize custom exceptions for experimental APIs

[verdan]

Make sure you have checked all steps below.

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title.

Description

• Here are some details about my PR, including screenshots of any UI changes:
  This PR adds custom exceptions for the APIs, so that we can handle different kind of exceptions efficiently. We can also use these custom exceptions to add the default static message for each exceptions instead of adding that repeatedly in the code. This also helps standardize the status code of each exception.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.

Code Quality

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[codecov-io]

Codecov Report

Merging #3496 into master will increase coverage by 0.02%.
The diff coverage is 89.15%.

@@            Coverage Diff             @@
##           master    #3496      +/-   ##
==========================================
+ Coverage    77.4%   77.42%   +0.02%     
==========================================
  Files         204      204              
  Lines       15185    15195      +10     
==========================================
+ Hits        11754    11765      +11     
+ Misses       3431     3430       -1

Impacted Files	Coverage Δ
airflow/api/common/experimental/get_task.py	100% <100%> (ø)	⬆️
airflow/api/common/experimental/trigger_dag.py	100% <100%> (ø)	⬆️
airflow/exceptions.py	100% <100%> (ø)	⬆️
...rflow/api/common/experimental/get_dag_run_state.py	100% <100%> (ø)	⬆️
airflow/api/common/experimental/pool.py	100% <100%> (ø)	⬆️
airflow/api/common/experimental/delete_dag.py	82.6% <100%> (-2.58%)	⬇️
airflow/www_rbac/api/experimental/endpoints.py	91.95% <80%> (ø)	⬆️
...rflow/api/common/experimental/get_task_instance.py	90% <80%> (ø)	⬆️
airflow/www/api/experimental/endpoints.py	90.14% <82.6%> (ø)	⬆️
airflow/models.py	88.16% <0%> (+0.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update eaa03db...b28e26a. Read the comment docs.

[verdan]

@jgao54 @Fokko can I please have some feedback on this PR?

[Fokko]

This looks good to me, one minor nit here. Bit hesitant with the Exceptions, if we have to create an exception for every possible resource, this can grow quite big. Airflow{Dag,DagRun,Pool}NotFound etc.

[Fokko]

Maybe add a docstring here

[verdan]

That's true, however, I've already added the NotFound exceptions for each of the resources in this PR, and this would help us standardize and differentiate the exceptions based on the objects.

[Fokko]

Thanks @verdan for cleaning this up! Cheers