match() method previously was able to change the error filed of a object.
That looked as excessive because pattern is compiled in constructor,
so to the moment of match() call should be already known if Regex state is valid or not.
match() returns a bool flag what should be enough to know was there some fail during the call or not.
This change allows to make match() to be const what is useful if we have constant reference to Regex
and want to call match().