[electrum] log: only log in verbose mode in CachedWalletPasswordWidget

Summary:
My intention was to log this message only in verbose mode, but it turns out print_msg always logs the message. We need to use print_error instead. This is a bit counterintuitive to me, as in other logging frameworks the severity of errors is higher than the severity of info messages and debug messages.

This antipattern of using print_error for debbuging logs will be cleaned out of the codebase when I eventually backport https://github.com/spesmilo/electrum/commit/3385a94753af2d93913ee9e9d67d18627c9c8f9e

Test Plan: repeat the test plan from D14729 with and without the -v command line option, verify that the message is only printed in verbose mode

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14765