[WebAssembly] Improve error messages regarding missing indirect function table. NFC

Use report_fatal_error here since this is an internal error, and not
something the user can/should be trying to fix.

Also distinguish between the symbol being missing and the symbol having
the wrong type.

We have a failure internally where the symbol is missing.  Currently
trying to reduce the test case to something we can attach to an llvm
bug.

Differential Revision: https://reviews.llvm.org/D99960