tests: improve logging on failure

It is helpful to print the error right at the point
of failure, rather than after the (potentially long
running) backtrace decode & log search jobs.

It'll get printed again later as well, but this way
I can search from the start of the file for the exception
name, and jump straight to the timestamp of the failure.

tests/rptest/services/cluster.py

@@ -34,10 +34,13 @@ def wrapped(self, *args, **kwargs):
             try:
                 r = f(self, *args, **kwargs)
             except:
+                self.redpanda.logger.exception(
+                    "Test failed, doing failure checks...")
                 self.redpanda.decode_backtraces()
                 self.redpanda.raise_on_crash()
                 raise
             else:
+                self.redpanda.logger.info("Test passed, doing log checks...")
                 if check_allowed_error_logs:
                     # Only do log inspections on tests that are otherwise
                     # successful.  This executes *before* the end-of-test