Improve error messages of PatrolFinder.first/last/at(index)

[bartekpacia]

Reported today by @jBorkowska, but it has been occurring since always. It's just the default Flutter behavior.

2 examples. Let's say that we're doing:

expect($(Button).first, findsNothing);

We'll get a StateError (Bad state: no element) instead of a failing assertion. We need to research how this can be improved.

Another problem is with await $('non-existing').scrollTo(). This will throw a StateError instead of our own, more descriptive exception.

PS There's also a problem with stack-trace not containing the file and line which caused the exception to be thrown. This is a Flutter bug and is separate from this issue.

[bartekpacia]

This was superseded by #1044.

[github-actions]

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