better error message for macros with MOD_SEPs
simplified msg (TMI on old one)
added test case for macros with separators error message
auto merge of #5231 : jbclements/rust/better-macro-error-message, r=graydon

Macro invocations with path separators (e.g. foo::bar!()) now produce a sensible error message, rather than an assertion failure. Also added compile-fail test case.

Fixes #5218 ?