review "assert" usage

See there for the problem description:

https://community.sonarsource.com/t/feature-python-assert-should-be-consider-harmful/38501

TL;DR: for now, do not run borg via `python3 -O` or with `PYTHONOPTIMIZE` set.

In the code, `assert` should be only used for:
- our test suite
- at places in production code where it doesn't really matter if the assert is executed or not. it must never be used if not executing the assert would influence correctness or security.
- if it matters, the assert should be replaced by `if : raise SomeException`

So, the task for borg master branch is to do a systematic review and fix all problematic asserts.

In case we find a lot of places to fix, a quick workaround for 1.4-maint branch could be to disallow running borg with assertions switched off, something like:

```
+try:
+ assert False
+except AssertionError:
+ pass # OK
+else:
+ print("Borg requires working assertions. Please run Python without -O and/or unset PYTHONOPTIMIZE")
+ sys.exit(2)
```

Note: 2 is the classic error code for a fatal error, but borg 1.4.x also supports modern exit codes, so an appropriate one (2 or more specific) needs to be returned for that.