Improve "user-defined expansions/condition not f[o]und" errors.

Right now there are two problems with these error messages:

1. They don't say which identifier wasn't found.
2. They say "not fund" instead of "not found"

(The span logic does not always indicate the error position, unless you are using nightly and -Zmacro-backtrace.)

I think that the right fix for 1 is to have DefinitionName implement Display, and to include its name in the error messages generated. Please let me know if that's reasonable and I'll do it.