Only show warning about QCA on user input.

When compiled with QCA support, merely trying to decrypt messages
should not warn about missing qca-ossl; this message should only be
shown on user input (such as the key-related commands) and incoming
messages really requiring qca-ossl to function (key exchange).
Show a message when failing to load the key file

People got confused when quassel did not accept their keyfile; now a
message is shown hinting at the possibility of a passphrase being
present.
Never use the expressions "allows to", "helps to", "requires to" etc. Each of these expressions requires a direct object.
fixes some  lintian warnings in debianoid systems