fileOrDefault method

[fsiler]

Consul Template version

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running the latest version, please upgrade before submitting an
issue.

consul-template v0.32.0 (a926131)

Expected behavior

The fact that the file function always returns an error if the file doesn't exist is a problem for me. I'd really like to have a fileOrDefault version as an analog of the keyOrDefault function.

Actual behavior

consul-template crashes if the file doesn't exist.