feature(Execution): Allow ignoring failed stages

[AbdulRahmanAlHamali]

Goes with:
spinnaker/deck#9039
spinnaker/gate#1452

Allow users to manually ignore failed stages. This is different from the usual FAILED_CONTINUE, in that the user typically would desire this to fail and stop the whole pipeline, but in some cases, they might review the reason of failure, or fix it externally, and in that case, they can ignore the failure.

[spinnakerbot]

The following commits need their title changed:

• 40ecffd: initial commit

• a1870d0: fix

• 17ccf31: fix

• 2d0b6f6: fix failed tests

• 4dc138e: fix exception

• 4b9d07b: fix error handling

• 4d05b2b: working

• 6aaa495: change all pipelines into running

Please format your commit title into the form:

<type>(<scope>): <subject>, e.g. fix(kubernetes): address NPE in status check

This allows us to easily generate changelogs & determine semantic version numbers when cutting releases. You can read more about commit conventions here.