Revamps structural error detection / handling.

Previously we'd only detect structural errors on the very first level.
This leads to incorrectly balanced braces not being discovered, and thus
incorrect indentation.

This change fixes the problem by:
- changing the parser to use an error state that can be detected
  anywhere inside the productions, for example if we get an eof on
  SOME_MACRO({ some block <eof>
- previously we'd never break lines when we discovered a structural
  error; now we break even in the case of a structural error if there
  are two unwrapped lines within the same line; thus,
  void f() { while (true) { g(); y(); } }
  will still be re-formatted, even if there's missing braces somewhere
  in the file
- still exclude macro definitions from generating structural error;
  macro definitions are inbalanced snippets