Compiler: should return error on invalid patterns

[shreyas-londhe]

The compiler should return error when trying to compile an invalid regex pattern. For example (\r\n|^)to:([^\r\n]+<)?[a-zA-Z0-9!#$%&\*\+-/=\?\^_{|}~.]+@[a-zA-Z0-9_.-]+>?\r\n`.