Improve better error handling for malformed regex expressions

See https://github.com/ColinEberhardt/assemblyscript-regex/pull/35

I haven't taken a terribly structured approach to error handling, hence there may be cases where the error reporting is poor or the behaviour unexpected. Furthermore, the [test suite I am using](https://github.com/ColinEberhardt/assemblyscript-regex/tree/main/spec) does't have many error cases.
