FastIgnoreRule: include bad pattern in log message

FastIgnoreRule: include bad pattern in log message

When a .gitignore pattern cannot be parsed include the pattern in the
log message. Just reporting "not closed bracket" isn't helpful if the
user doesn't know in which pattern the problem occurred.

Even better would be to include the full path of the .gitignore file
that contained the offending pattern. This is not implemented in this
change; it may need new API and needs more thought.

Bug: 571143
Change-Id: Id5b16d9cf550544ba3ad409a02041946fa8516ab
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>