Use a custom exception message when failing to get image size

[caendesilva]

Fixes an embarrassing bug in error output, that I missed due to PHPUnit (for some reason) using substring comparisons for testing exception messages.

This lead to the following:

File [Image at _media/foo does not exist] not found.

This uses the new custom message feature from #1398 so it instead looks like this:

Featured image [_media/default] not found.

[codecov]

Codecov Report

Merging #1399 (135f77e) into master (eb1f272) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             master    #1399     +/-   ##
===========================================
  Coverage     99.97%   99.97%             
- Complexity     1705     3410   +1705     
===========================================
  Files           180      360    +180     
  Lines          4619     9238   +4619     
===========================================
+ Hits           4618     9236   +4618     
- Misses            1        2      +1     

Files	Coverage Δ
...amework/Features/Blogging/Models/FeaturedImage.php	100.00% <100.00%> (ø)

... and 180 files with indirect coverage changes