Improve proc macro errors a bit

Distinguish between
 - there is no build data (for some reason?)
 - there is build data, but the cargo package didn't build a proc macro dylib
 - there is a proc macro dylib, but it didn't contain the proc macro we expected
 - the name did not resolve to any macro (this is now an
 unresolved_macro_call even for attributes)

I changed the handling of disabled attribute macro expansion to
immediately ignore the macro and report an unresolved_proc_macro,
because otherwise they would now result in loud unresolved_macro_call
errors. I hope this doesn't break anything.

Also try to improve error ranges for unresolved_macro_call / macro_error
by reusing the code for unresolved_proc_macro. It's not perfect but
probably better than before.

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