fix(dockerimage,transformers) improve validation failure cases

[dduportal]

Fix #432

With updatecli 0.18.3 and 0.19.0, the dockerimage condition already fails thanks to #418 .
But this PR improves the user feedback when a dockerimage condition is provided with an empty tag.

It also fails transformers when they are applied with an empty input source (surprise bugfix!).

Finally, this PR ensure that a failing transformer does not duplicate the error message 3 times :)

Test

To test this pull request, you can run the following commands:

make test
make test-short
make test-e2e

Additional Information

• Result with the failing condition in Docker Image Condition should fails if the source is an empty value #432 case:

CONDITIONS:
===========

checkDockerImage
----------------
ERROR: Condition validation error for the image "cytopia/yamllint": source input is empty and no explicit tag is specified.

✗ condition not met, skipping pipeline

• Result if the condition is removed (e.g. target's transformer failing due to empty input value):

⚠ empty source returned


TARGETS
========

show result
-----------
ERROR: Something went wrong in target "show result" :
ERROR: Validation error: input for transformer is empty.


=============================

REPORTS:


✗ UPDATECLI.YAML:
        Sources:
                ⚠ [lastRelease] Get latest yamllint version(githubRelease)
        Target:
                ✗ [show result]  Update yamllint version in PodTemplates.yaml(shell)