Need to safely handle case where bitmap fails to load

[benjiwheeler]

If the bitmap loading in scratch-vm/src/import/load-costume.js's fetchBitmapCanvas_() function fails, we don't return any value from the promise.

This may be causing an error in loadBitmap_(), where we attempt to access the fetched.assetMatchesBase property without considering that fetched may never have received any value.

Example of a project that may be hitting this problem: https://scratch.mit.edu/projects/318191515/

[apple502j]

Bump, see also https://scratch.mit.edu/discuss/post/5207123 (reproable in Chromium-based browser, not in Firefox. Is this another strange bug?)