Add a flag to just warn about missing script files

[deluan]

When devs are working in parallel in the same dev space, when one dev applies a script to the space, other devs can gets an error, because the script is not in their local workspace yet.

  error : Error: Missing migration file: 20180227004930-add-field-to-block-promo.js

There should be a way to make this error a warning instead, so it does not break any dev's workflow

[mattwexa]

👍 @deluan
We have the same need, what can we do to help?

[mattwexa]

Sorry to bother you again @deluan, to restate we are willing to contribute the code to add an option (like warn-if-missing-scripts or warn-missing or something) and catch any missing script error and output a warning instead if the option is given on the command line. What do you think?