Expand set of errors caught in set_context().  Some new errors, some
old error messages changed for consistency.

Fixed error checking in generator expression code.  The first set of
tests were impossible condition given the grammar.  In general, the
ast code uses REQ() for those sanity checks.

Fix some error handling for augmented assignments.  As comments in the
code explain, set_context() ought to work here, but I got unexpected
crashes when I tried it.  Should come back to this.

Add note to Grammar that yield expression is a special case.

Add doctest cases for SyntaxErrors raised by ast.c.