SEMI recommendation api: add method to check validity of input method declaration and create error codes

[Katya Garmash]

Why

The current implementation #121 takes as input a list of strings, assuming it is a valid method declaration.

What we want

Add a subprocedure in ``recommend_for_method'' checking it is a valid method declaration. If it is not valid, the API should return an error code.

Proposed solution

• Validity checks:
  • check it is not a class declaration
  • check for bracket balance
  • part of the errors can be caught by try-catch wrapper around procedure creating AST tree from string.
• Error codes: TODO

@acheshkov @lyriccoder