Add error message for clients that lack a required crypto library.

TUF's crypto dependencies was recently changed: ED25519 signatures supported by default, and require PyCrypto or tuf[tools] to verify RSASSA-PSS signatures.
The error returned for clients that tried to verify rsassa-pss signatures without the required library was bad.  Error message returned now:

$ basic_client.py --repo http://localhost:8001
Error: No working mirror was found:
  localhost:8001: Metadata downloaded from the remote repository specified an RSA signature.  Verifying RSA signatures requires PyCrypto.
$ pip install PyCrypto, or pip install tuf[tools].
Merge pull request #197 from vladimir-v-diaz/develop

Add error message for clients that lack a required crypto library.