[BUG #6628] uniformly raise SyntaxException in the parser

- catch AssertionError and re-raise it as SyntaxError (that's what the assertions
  in treegenerator are about)
- made parsing of map keys more strict (was: expression;
  is: number/string/identifier)
[BUG #6628] lint job now logs syntax error and continues
[BUG #6628] minor improve error logging