[backport#15491] wallet: Improve log output for errors during load

Summary:
faf369880 wallet: Improve log output for errors during load (Glenn Willen)

Pull request description:

When loading the wallet, display the entire path in error messages, instead of
the name (which, for the default wallet, is the empty string.)

When an exception occurs during wallet loading, display e.what() if possible,
instead of nothing.

https://github.com/bitcoin/bitcoin/pull/15491/commits/faf369880822188f2738c6f046d9ef14c9585713

Backport of Core PR15491

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7028