Include more information when json parsing fails

BUG=

Review-Url: https://codereview.chromium.org/2993643002
Cr-Mirrored-From: https://github.com/luci/luci-py
Cr-Mirrored-Commit: 82ffcaeb581f377c0b5059e934e81d28d5e8a800
diff --git a/isolated_format.py b/isolated_format.py
index e71a4b5..6dc7336 100644
--- a/isolated_format.py
+++ b/isolated_format.py
@@ -436,8 +436,8 @@
   """
   try:
     data = json.loads(content)
-  except ValueError:
-    raise IsolatedError('Failed to parse: %s...' % content[:100])
+  except ValueError as v:
+    raise IsolatedError('Failed to parse (%s): %s...' % (v, content[:100]))
 
   if not isinstance(data, dict):
     raise IsolatedError('Expected dict, got %r' % data)