Show parse errors from looked up env vars

[wallyqs]

Currently if there are issues parsing an env var, the config parser fails silently.

• Related to Values that begin with a number cannot be resolved from the environment #885
• Documentation added (if applicable)
• Tests added
• Branch rebased on top of current master (git pull --rebase origin master)
• Changes squashed to a single commit (described here)
• Build is green in Travis CI
• You have certified that the contribution is your original work and that you license the work to the project under the Apache 2 license

/cc @nats-io/core

Signed-off-by: Waldemar Quevedo wally@synadia.com

[derekcollison]

LGTM

[coveralls]

Coverage decreased (-0.003%) to 91.7% when pulling d0d0525 on wallyqs:lookup-env-errors into a4741c5 on nats-io:master.