[pavog]

Based on #36

When preg_match_all() returned false, for example because a pattern exceeded the backtrack limit, PatternAnalyser treated it like a pattern that doesn't match.
Such a pattern silently never found its problem, and nothing showed that it was broken.

findMatches() now triggers an E_USER_WARNING that names the pattern and the reason.
For example:

    preg_match_all() failed for pattern /I have a problem with (\w+)|(?:a+)+b/: Backtrack limit exhausted

The pattern is still treated as not matching, so the rest of the log is analysed as before.

This builds on #36, because the check lives in findMatches().

Decisions

• A warning and not an exception: PHP itself reports an invalid pattern with a warning and false, and one failing pattern should not stop the analysis of the whole log.
• The message names the pattern and not the insight class, because findMatches() only knows the pattern and the text.
• Matches that were found before the pattern failed are discarded, as before.
• An invalid pattern now produces two warnings, the one from PHP and this one.
  Only this one contains the pattern.

Changes

• trigger a warning in PatternAnalyser::findMatches() when preg_match_all() fails
• test that a failing pattern is reported and the other patterns and entries are still analysed
• test that matches found before a pattern failed are discarded

Not covered

• Projects that use codex only fail their tests on such a warning if PHPUnit runs with failOnWarning.
  Their current test logs don't trigger it.
• An application that converts warnings to exceptions aborts the analysis when a pattern fails.