Ensure 'run_tests.sh local' fails correctly when gometalinter errors.

[jpz]

For a clean install, gometalinter can return non-zero, or gometalinter
may not run at all if it is not present.

In these cases, the content of stdout is zero bytes, and the script
continues when it should error out.

This error can easily occur when initially setting up the dev project
and one has not installed gometalinter or unconvert. (This is not
a problem for docker testing, as these are preinstalled.)

Also updated README.md to describe the testing dependencies.

[davecgh]

Thanks for the PR. I wonder if it should just be gometalinter --install? That will instruct gometalinter to install all of its linters. That way, if any now ones are enabled, the docs wouldn't have to be changed. The tradeoff of course is installing extra things that aren't currently used.

[jpz]

Thanks for feedback - gometalinter still needs to be installed, but I've put this in the run_tests.sh

After looking at the contents of the Dockerfiles, I briefly considered the merit of writing a initial_dev_setup.sh script which replicated that, so we could pare down the manual instruction in README.md, but making this smaller change seems sufficient for the moment.