fixed access to tx.error in case tx is None
Merge pull request #898 from Tafelpoot/tx_fix

fixed access to tx.error in case tx is None
simplify error handling during transaction signing
use monospace font in tx window. fixes #897
Do not use XDG_DATA_HOME. Tell user to use virtualenv if they are not root
fix for virtualenv install
fix hardware plugins with tx.error
trezor: raise and catch BaseException in signing
trezor: don't call sign_transaction twice