Assert and stack traces for Bash scripts

[cameel]

While working on #12141 I was really missing a quick way to assert stuff and the lack of a proper stack traces when a script fails makes debugging really annoying. This PR adds a rudimentary helper that can print a stack trace and a function that we can get an assert-like failure.

[cameel]

Initially I implemented a full assert that you would invoke like this:

assert "[[ $INTERACTIVE == false ]]"

It could print the condition if you did not provide a message, which was nice.

The problem is that it required eval to check the condition and also that quoting was tricky - you had to quote the whole condition with "" (otherwise the variables would be evaluated inside the assert rather than in the current function) and then use '' over any variable used inside that could evaluate to an empty string.

In the end I thought that just a simple assertFail that prints a stack trace is enough. It still clearly indicates that the condition you check is an assertion and condition evaluation has no quirks (at least not more than usually in Bash). Also, the condition itself can be seen in the stack trace.