Throw error message on resizing image and prevent hard fail

[monishdeb]

Mosaico uses the Intervention library to handle image manipulation. we've found a few instances where this throws a fatal error and breaks the application, when we think it should log an error to file, perhaps throw a UI warning -- but not block use of the system. Two cases where it causes hard failure:

1. Files in the images/uploads folder (i.e. the Mosaico gallery) had improper permissions, and
2. when there were files there that were not valid image files.

BEFORE

Hard failure breaks the design screen

AFTER

Capture the error messages and notify the user on compose screen.

[lcdservices]

This works as expected. The UI can still be used.

[eileenmcnaughton]

Seems OK to me - @mattwire ?

[mattwire]

This seems sensible and has been tested by @lcdservices