Provide helpful error messages when AESM plugins are missing

[jethrogb]

In recent versions of AESM, individual AEs are served by plugins, which may or may not be installed. So just checking if AESM is running is not sufficient, you might still get a “Service Unavailable” response later when trying to do a certain request. For example, the quoting enclave plugin may be installed but not the launch enclave plugin.

In addition to making aesm-client's errors more useful in this regard, sgx-detect should clearly display which pieces are missing. sgx-detect may even offer to install the missing pieces.