validate_failures.py: Print message in case of broken sum file error

Provides some context to help debug the problem.

Change-Id: I78328cec1edd04cfc896d3facbc39f8e02b36d9c
diff --git a/contrib/testsuite-management/validate_failures.py b/contrib/testsuite-management/validate_failures.py
index 78c43f9..5d6ea24 100755
--- a/contrib/testsuite-management/validate_failures.py
+++ b/contrib/testsuite-management/validate_failures.py
@@ -137,6 +137,7 @@
       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}"')
         raise
       self.tool = tool
       self.exp = exp