Checks that exported symbols are valid C identifiers. Refs #800.
Addresses issues raised on #947. Refs #800.

* Uses errGenerated instead of deprecated extending of enums.
* Reduces bloat and usefulness of end user error messages.
* Limits checks to C, Cpp and Objc targets.
Merge pull request #947 from gradha/pr_checks_bad_exported_symbols_800

Checks that exported symbols are valid C identifiers. Refs #800.