Server-side validation of user credentials

[kaloyan-raev]

The Import Keys feature does not check if the provided credentials are actually valid on the Bridge.

The following 2-step validation should be done in addition to the current client-side validation:

1. List buckets.
   -- if error code 200 or 304 is received then user and password match
   -- If error code 401 is received then user and password does not match
   -- in all other cases - unknown error
2. Check if the listed buckets can be decrypted with the provided encryption key
   -- if at least one bucket is decrypted successfully then the encryption key is accepted as valid
   -- if none of the buckets can be decrypted then the encryption key is invalid
   -- if no buckets in the account - the encryption key is accepted as valid

[kaloyan-raev]

This is now available in v0.8.