Fix logException message building

Fix psr/log dependency, bump to PHP 8.0 minimum (#5)

Support PHP 8.1, bump minimum to 7.3 (#4)

Move vectorface/cache dependency to dev

Only needed for FakeMemcache in tests, not needed at runtime.

Cleanup, bump required version to 7.0

Improve the logException method slightly.

Get rid of another condition.

Some improvements based on scrutinizer suggestions.