Access correct key for template name

When an exception occurs while loading the config agent driver, the
handler tries to log a message, but accesses the wrong key to get the
template name. This causes another exception, which masks the original
exception.

This change accesses the correct key and performs logging inside a
with block to (defensively) preserve the exception context.

Change-Id: I3991bc4c509bbd8d68f7a1c8c8314404f2e0eafa
Closes-Bug: #1368900
Merge "Access correct key for template name"