fix: irma server now shows error message when invalid requestor json is passed using flag or env var

Fixes #38

Fixes #37

In particular, combined issuance sessions in which 2 or more attributes are
requested from one particular credential type, due to a miscalculation during
verification of the attributes.

Fixes #36

feat: Support --url for session. Some code duplication was unavoidable without splitting up requestorserver.initialize

When disclosing two attributes from the same credential instance, irmaclient would disclose both attributes twice.

irma scheme verify now errors if an issuer's latest public key does not support sufficient attributes

Verifying an individual scheme no emits the same warnings and errors as verifying multiple schemes.