Better error messages when plugin is not found

[zachgersh]

Depends on #121

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Previously when the plugin failed to be found the error message would be unhelpful:

mynet : error executing ADD: could not find "." plugin

Now, error message looks like

mynet : error executing ADD: failed to find plugin "some-plugin" in path [/gopath/src/github.com/appc/cni/bin]

• Added tests in the ginkgo style (which means ginkgo / gomega are godep'ed)
• Tests are now run as part of the testing script
• Testing style of /pkg/invoke is now much more descriptive + covers edge cases
• ginkgo / gomega style tests still work with the regular old go test command

[steveej]

Thanks @zachgersh for these test cases. This work is highly appreciated!

Please make the addition of ginkgo / gomega deps a separate commit preceding all commits that make use of the new packages.

[zachgersh]

Thanks @steveej! I am actually going to pull ginkgo / gomega out of this PR. If you would be so kind as to merge #121 first then those dependencies will be added and these tests should just work!

Really appreciate the quick feedback.

[zachgersh]

@steveej this commit is now ready. I will add the tests being run back in once both the PRs I have outstanding have been merged 👯