volume: improve test coverage and minor refactoring

The volume package does not test enough use-cases.

Improve test coverage by adding additional tests and refactoring
current tests to use table testing.

This change introduces a new error var to make testing unsupported
volume type errors easier.

This change does not introduce any changes in behavior.
Merge pull request #649 from kelseyhightower/improve_volume_test_coverage

volume: improve test coverage and minor refactoring