Fix #614. Cause by 8215623.

[Matafou]

The regexp change in 8215623 to support "Fail" command had to be
refined to capture correctly some error messages.

[Matafou]

This fix is a bit fragile but we need to fix error highlighting quickly. So I will merge this soon.

For the record and a future better implementation:

The fragility comes from the fact that the <P>-error-regexp is used in two ways: either on the string <PA>-last-output or directly in the response buffer with re-search-forward. I think it would be cleaner if used only on the string.

Once it is only done on the string, we could have an optional <PA>-match-error-p function which would take precedence over <PA>-error-regexp taking a string and testing if it contains an error message. It would allow for a better fix for #597, avoiding negative regexp.

No hurry though.

[Matafou]

The bug #612 has nothing to do with this PR. I merge.