[electrum] fix error dialogs in main_window

Summary:
util.is_verbose does not exist since https://github.com/Bitcoin-ABC/ElectrumABC/commit/fdefdbc8f5deb70b92efa2341bb83910d94c7f3c
As a result, the user sees a python traceback instead of a helpful error dialog.

In change_password_dilaog, an except branch was unreachabe, merge it with the previous branch (this has been broken since 2016 https://github.com/spesmilo/electrum/commit/c6a46df15887736969c7a7a448c76f9a9ed0c21e)

Test Plan: In the application, load a transaction from text, type a malformed transaction (non-hex, non-JSON), check that the error dialog prints "Electrum ABC was unable to parse your transaction". Check that the error stack is now only displayed in the terminal when running in verbose mode -v.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14531