feature(Execution): Allow ignoring failed stages

[AbdulRahmanAlHamali]

Goes with:
spinnaker/gate#1452
spinnaker/orca#4113

[spinnakerbot]

The following commits need their title changed:

• ab55132: allow ignore failure

• 0a259a0: add the directive

• eda3856: import module

• f6751a2: add ignore action

• c6db1f0: fix reason

• 88cb6a4: fix display of details

• 2b7662f: optional display of exception

Please format your commit title into the form:

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

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