Make sure that a pubkey is only appended to the checkpub array if it
corresponds to a change address. Signing will fail otherwise, if a
non-change pubkey is sent as part of the checkpub list to the Digital
BitBox.
use common cross-thread HTTP method, which is put in network.py,
since that is where the proxy is. TrustedCoin tested successfully,
but DigitalBitbox can't be tested completely due to #4903
before this commit, digitalbitbox would not use any proxying
we should not raise generic Exception when wanting to communicate with
the user. it makes distinguishing programming errors and messages hard,
as the caller will necessarily need to catch all Exceptions then