(#5838) Refactored error handling logic into find_in_cache.

Moved the error handling logic surrounding the call to
Puppet::Indirector::Indirection#find_in_cache() into the
find_in_cache() method itself, so that won't need to be duplicated
when find_in_cache() is called from elsewhere.

Paired-with: Jesse Wolfe <jesse@puppetlabs.com>