Fix replacing Horizontal Tab with the unicode char

Commit 4676ff82af669595edaf090c97a28161d67782a1 caused horizontal tabs
to be replaced with the HT character of the unicode control char block.
Fix that by replacing it with 8 spaces again.
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.