In case of a user-defined RAPIDJSON_PARSE_ERROR_NORETURN that throws
an exception instead of using the Rapidjson ParseError API, the early
return paths performing the stack unwinding manually can be omitted as
well.

This patch provides a customizable RAPIDJSON_PARSE_ERROR_EARLY_RETURN
macro to remove these (then unneeded) control paths from the parsing
implementation (with and without a return value).

Secondly, clearing the parse stack is moved to a small helper struct
that calls stack_.Clear() from its destructor.  This avoids the need
for the 'goto' in the ParseStream function and ensures proper cleanup
even if e.g. a user-defined Allocator throws an exception.