rake locale:plugin:find fixes

[mzazrivec]

Two fixes:

• use args[:engine] rather than args.key?(:engine) -- for some reason the later would always return nil.
• for non-existent plugin (e.g. type), print nice error rather than an ugly traceback.

[miq-bot]

Checked commits mzazrivec/manageiq@f126582~...1d31c8b with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
1 file checked, 0 offenses detected
Everything looks fine. 👍