[autotest] Find package of crashing processes.

Do some clever find/portageq tricks to map an executable name back to
the package that owns the executable while we're collecting crashes.
Follow up work will then deduplicate and file bugs on these crashes.

BUG=chromium-os:32834
TEST=pkill -SEGV tlsdated, and looked for "Would report bug on
net-misc/tlsdate-0.0.4-r14." in logs.

Change-Id: I54426eaef6b1954746cb3d2405a0c292a4979559
Reviewed-on: https://gerrit.chromium.org/gerrit/29828
Commit-Queue: Alex Miller <milleral@chromium.org>
Reviewed-by: Alex Miller <milleral@chromium.org>
Tested-by: Alex Miller <milleral@chromium.org>
diff --git a/client/common_lib/error.py b/client/common_lib/error.py
index 2783fd1..d52eb14 100644
--- a/client/common_lib/error.py
+++ b/client/common_lib/error.py
@@ -570,6 +570,11 @@
     pass
 
 
+class NoUniquePackageFound(Exception):
+    """Raised when an executable cannot be mapped back to a single package."""
+    pass
+
+
 # This MUST remain at the end of the file.
 # Limit 'from error import *' to only import the exception instances.
 for _name, _thing in locals().items():