Display proper messages if user credentials are not authorized

[le-git-imate]

Description of issue or feature request
le-git-imate assumes that imported account info is correct. Talking to the server, the user is authorized using the stored data (i.e., token or password). However, the authorization may fail. In this case, le-git-imate should notify the user about the issue. Depending on how we address #4, the message may be displayed in different steps.
This issue may be addressed in accordance with #14, #26.

Current behavior

• No proper message is shown when the authorization fails.

Expected behavior

• Display a message when user is not authorized to call and API or preform the git-pack protocol.