Warn or error on mis-cased Procfile

[edmorley]

On Heroku currently, Codon outputs the following warning if an app has a procfile rather than the correctly named Procfile:

~     Mis-cased procfile detected; ignoring.
~     Rename it to Procfile to have it honored.

We should add something similar to this buildpack.

We should likely also make this an error rather than a warning.

[edmorley]

An example of a lower-cased procfile causing confusion even on classic (where there is already a warning):
https://heroku.support/1414816

As such it seems either an error or else accepting both capitalisations might be preferable?

[schneems]

accepting both capitalisations might be preferable?

Seems good. We could downcase everything so PrOcFiLe would be accepted as well.

Kick it up a notch: When there's no match output the closest 3 Levenshtein distance filenames