Include env var in validation error message

[philbooth]

We'd find something like this helpful for debugging validation errors on process startup, since we're mostly working with environment variables rather than config files.

Would it, or something like it, be okay to land?

[coveralls]

Coverage remained the same at 100.0% when pulling e101029 on philbooth:validation-message-env-var into be350a7 on mozilla:master.