[FEA] Improve regexp error messages

[andygrove]

Is your feature request related to a problem? Please describe.
Some of the error messages thrown by the transpiler, such as "nothing to repeat", are not very helpful to end-users. It would be better to throw more specific errors, such as "cannot support choice with empty left-hand side". It would be nice to include the position within the regular expression pattern as well.

Describe the solution you'd like
See above.

Describe alternatives you've considered
None

Additional context
None