Add incorrectly named files as potential problem for ModuleNotFound e…

[oniskanen]

…rror

This will help debug elm-make error when importing a module MyModule and the filename isn't MyModule.elm

Example of the current error where this addition might help:

elm-css src/main.elm
I cannot find module 'MyModule'.

Module 'Main' is trying to import it.

Potential problems could be:
  * Misspelled the module name
  * Need to add a source directory or new dependency to elm-package.json
( * File name does not match the module name )

[process-bot]

Thanks for the pull request! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[alex-tan]

Seems like "File name does not match the module name" should replace "Misspelled the module name" as they're the same thing, right?