Wishlist: Propagate errors, don't just exit

[BenWiederhake]

Specifically, tglmp_on_start (when no valid RSA keys were found) in mtproto-client.c:1305

As there's no feedback from tgl about whether loading the key worked or not, the wrapping application has to duplicate lots of checks (e.g. key size, exponent sanity, etc), which is brittle and unreliable.

Suggested resolutions:

• tgl_set_rsa_key{,_direct} return a "boolean", and tglmp_on_start just asserts that at least one key exists (because then it would be a programmer error if it doesn't).
• tglmp_on_start returns a "boolean"
• both of the above

In an unrelated note, there's also rpc_execute which makes a direct call to exit (when an invalid state/response pair is detected) in mtproto-client.c:1207.