Better error message when running class-dump on a static library

When running class-dump on a static library, the error message is very confusing:
>  Error: File doesn't contain the specified architecture (armv7). Available architectures are armv7, armv7s, i386.

With this fix, the error message is much less confusing:
> Error: Fat file doesn't contain a valid Mach-O file for the specified architecture (armv7).  It probably means that class-dump was run on a static library, which is not supported.