Only set err_info on an error, and use g_strdup_printf().

If the goal is to look like a libwiretap routine, we only need to set
*err_info on an error, and we should set it to a g_mallocated string (or
NULL).

Handle WTAP_ERR_UNSUPPORTED while we're at it - we never return it, but
we never return WTAP_ERR_UNSUPPORTED_ENCAP, either, but we handle it.

Change-Id: I9d93c43278d22f0fa77ec1cf7f29b476c8dd0dd0
Reviewed-on: https://code.wireshark.org/review/4565
Reviewed-by: Guy Harris <guy@alum.mit.edu>