Report all swarming and isolate fatal errors in a consistent way.

utils/tools.py

Index: utils/tools.py
diff --git a/utils/tools.py b/utils/tools.py
index eecf709641bc8497a3d8e57480a65b3a8071d029..2885954faca8a2040e4ec9f04fc5779f45ba774d 100644
--- a/utils/tools.py
+++ b/utils/tools.py
@@ -10,6 +10,7 @@ import optparse
 import os
 import sys
 import time
+import traceback
 
 
 class OptionParserWithLogging(optparse.OptionParser):
@@ -112,3 +113,23 @@ def fix_python_path(cmd):
   elif out[0].endswith('.py'):
     out.insert(0, sys.executable)
   return out
+
+
+def report_error(error):
+  """Prints a error to stderr, wrapping it into header and footer.
+
+  That way errors can be reliably extracted from logs. It's indented to be used
+  only for non recoverable unexpected errors. Is should NOT be used for input
+  validation, command line argument errors, etc.
+
+  Arguments:
+    error: error message string (possibly multiple lines) or an instance of
+           Exception subclass. In the later case a traceback will also be
+           reported. It's assumed that |report_error| is called in an except
+           block where |error| was caught.
+  """
+  print >> sys.stderr, '[------ Swarming Error ------]'
+  print >> sys.stderr, str(error)
+  if isinstance(error, Exception):
+    print >> sys.stderr, traceback.format_exc(),
+  print >> sys.stderr, '[----------------------------]'