handle bad distribution tarballs more gracefully

- If mysqld is not found, or not executable, be more verbose
  about the error.  Previously if mysqld was not executable an
  error message was logged indiciting it might be an
  incompatible tarball (i.e. 64-bit on 32-bit or 32-bit on an
  incompatible 64-bit system).  This change also logs an error
  if mysql doesn't exist - often due to using a source tarball
  rather than a precompiled binary tarball
- Resolves issue #46