FX History rates follow-up: Be more tolerant of cache file write errors
FX History rates follow-up: Be more tolerant of cache file write errors

This commit allow us to be more tolerant of cache file write errors.

If for some reason the user's filesystem is out of space or read-only,
still proceed with the history rates call path and update rates in GUI,
even if we fail to cache the file to disk.

fyookball/electrum,fyookball/electrum,fyookball/electrum