validate_failures.py: Print error messages to stderr

The script's stdout is usually redirected to a file, so error messages
shouldn't go there.

Change-Id: Ib0b57ebca1ceb18bdf4203f770451af5d9930ccd
diff --git a/contrib/testsuite-management/validate_failures.py b/contrib/testsuite-management/validate_failures.py
index 5d6ea24..824ba65 100755
--- a/contrib/testsuite-management/validate_failures.py
+++ b/contrib/testsuite-management/validate_failures.py
@@ -131,13 +131,15 @@
          self.name,
          self.description) = _VALID_TEST_RESULTS_REX.match(summary_line).groups()
       except:
-        print('Failed to parse summary line: "%s"' % summary_line)
+        print('Failed to parse summary line: "%s"' % summary_line,
+              file=sys.stderr)
         raise
       self.ordinal = ordinal
       if tool == None or exp == None:
         # .sum file seem to be broken.  There was no "tool" and/or "exp"
         # lines preceding this result.
-        print(f'.sum file seems to be broken: tool="{tool}", exp="{exp}", summary_line="{summary_line}"')
+        print(f'.sum file seems to be broken: tool="{tool}", exp="{exp}", summary_line="{summary_line}"',
+              file=sys.stderr)
         raise
       self.tool = tool
       self.exp = exp