This was inconsistent with the other error messages. This is useful as
it allows easy detecting of the offending item for editor plugins by
simply searching in the message for quotes.

As '*' does not appear in __future__.all_feature_names, this
also reports an error on : from __future__ import *

Also detect unused star imports.

This is a SyntaxWarning on Python 2.6+

Resolves lp:848467

This includes return and yield outside of a function and break and
continue outside of a loop. Fixes lp 1293654.

The problem is that these SyntaxErrors are not encoded in the ast
grammar, so they are not detected when just compiling to ast. You must
compile down to bytecode to catch them. The advantage here is that we
can still check for other kinds of errors in this case, because the ast
is still valid.