[tests] Report an error when .apk tests crash

[jonpryor]

While reviewing PR 445 I noticed that it said that 41 tests had
been run, and 0 failed. Compare to e.g. PR 474, which has 164
passing tests.

Something isn't right here.

Further investigation showed that for PR 445, execution on the
emulator had crashed ("Segmentation fault"). The error was ignored.

Ignoring errors was in fact By Design™; see commit 6358a64.

So what we need is for the RunUnitTestApks target to not report
errors, while still reporting errors.

Solve this conundrum by updating the <RunInstrumentationTests/> task
so that when the nunit2-results-path value can't be found, it
reports a successful run, and sets the
RunInstrumentationTests.FailedToRun property to the name of the
component that failed. This allows the ReleaseAndroidTarget target
to check for any failures, and report an <Error/> after shutting
down the emulator instance.

This should allow the build to "fail" when nunit2-results-path can't
be found -- a sign that the .apk unit tests didn't finish executing
-- while also allowing the emulator to be shut down.

Aside: xbuild doesn't support ContinueOnError="ErrorAndContinue",
nor does it set <Output/> properties whenever a task fails.
Consequently, the <RunInstrumentationTests/> task must return true
when nunit2-results-path isn't found, because there's no other way
to convince xbuild to collect the <Output/> values.