Update PipelineErorr typespec

[hubertlepicki]

So in order to fail a stage we can raise an exception or return an {:error, whatever} tuple or just :error I believe too.

In the first case this exception will be caught and indeed PipelineErorr's :error field will contain a struct of the exception.

If you, however, fail the stage by returning error tuple, the field will contain whatever you returned.

The same situation also happens when you fail a check by returning false. In that case we'll see something like: %Opus.PipelineError{error: :failed_check_verify_signature}, i.e. atom and not a struct.

I don't think we can predict what type precisely will be there, so I think the best is to return any?

[coveralls]

Pull Request Test Coverage Report for Build 116

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.065%

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Totals
Change from base Build 115:	0.0%
Covered Lines:	106
Relevant Lines:	107

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💛 - Coveralls

[zorbash]

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