[JENKINS-22859] Don't fail revprop validation as easily

This does not *fix* the issue (I don't know how), but it makes it far less likely to occur.

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Only print an error if both of the following are false:
* Field is empty (option 'exclusion revprop' unused)
* Error is authentication failure (there's other fields for that)

The first is a bit shaky due to the global exclusion revprop feature,
but chances are you're not going to see that 'not supported' error
anyway when not explicitly using a per-project setting.