Improve error handling

[lovasoa]

Currently, when parse fails, the user only gets a very uninformative error string as a result.
It would be nice to have a custom error type with useful functionality , such as knowing where the error happened.

[lovasoa]

See lovasoa@a0e27ac

[ms705]

Yes! I've been meaning to add support for more descriptive syntax errors for a while, but never got around to it. Independently, I believe nom's story for parse error handling has also changed, however, so it might be best do to this alongside a port to nom v4.

[ms705]

(Though I like the changed you linked; we can certainly merge that one right away.)

[tomharmon]

To add to this issue: Specifically we should use nom's VerboseError to improve error messages. One thing pointed out by @spazm is that in src/create.rs, the creation function, might want to through a nom error with descriptive error message rather than the existing assert!(table.alias.is_none())