Use Logging.warn to output warning messages

[gregod]

Using println for the warning messages in src/file.jl outputs directly to stdout. This creates issues when using a julia script in an unix pipe writing data to stdout.

This pull request replaces these calls with the warn method from the stdlib logging framework. They correctly attach to stderr instead, preventing the mix of messages and data.

[codecov]

Codecov Report

Merging #728 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #728   +/-   ##
=======================================
  Coverage   84.75%   84.75%           
=======================================
  Files          10       10           
  Lines        1850     1850           
=======================================
  Hits         1568     1568           
  Misses        282      282           

Impacted Files	Coverage Δ
src/file.jl	94.70% <100.00%> (ø)

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[quinnj]

@oxinabox, @bkamins, does this look good to you two? (or anyone else have an opinion here?). Seems fine to me.

[oxinabox]

this seems like a good idea.
Also using @warn makes it possible to use the rest of the logging infastructure to mute it, etc