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 e1010297455967069dfbb24734eab05d22804c07 on philbooth:validation-message-env-var into be350a7ded016543b3eea16953be1f81c4bf34be on mozilla:master.