Don't try to compile a pattern known to be in error

Don't try to compile a pattern known to be in error

Regular expression patterns are parsed by the lexer/toker, and then
compiled by the regex compiler.  It is foolish to try to compile one
that the parser has rejected as syntactically bad; assumptions may be
violated and segfaults ensue.  This commit abandons all parsing
immediately if a pattern had errors in it.  A better solution would be
to flag this pattern as not to be compiled, and continue parsing other
things so as to find the most errors in a single attempt, but I don't
think it's worth the extra effort.

Making this change caused some misleading error messages in the test
suite to be replaced by better ones.