Parser Improvement: Better error messages

[achreto]

Right now we rely on the NOM parser components to produce an error message. This work OK-ish but by times this could be improved through custom error messages.

Current State

This is the current error message. Indicating that it has expected a : but found a ) this is fine, it tells us what's needed on a token level.

error: expected `:`, found `)`
     --> tests/vrs/methods/methods_05_arguments_no_types.vrs:3:15
      |
    3 |      fn foo(bar) -> bool
      |                ^ help: add `:` here.

Desired Output

An improved error message would tell the user that the type information is missing, for example:

error: expected `type spec`, found `)`
     --> tests/vrs/methods/methods_05_arguments_no_types.vrs:3:15
      |
    3 |      fn foo(bar) -> bool
      |                ^ help: add `: type` here.

There are more such examples through out the parser.