PluginManager: tests for missing plugin name when passing --url

Adding code to test for unset plugin names to fail fast with descriptive error messages. Also simplified the series of `if` statements checking for the commands by using a `switch` (now that it's using Java 7), added tests, and updated random exceptions with the up-to-date flag names (e.g., "--verbose" instead of "-verbose").

Closes #5976.
Closes #6013.