Media: Improve wording for error message on failed post-processing of uploaded images.

Props eclev91, desrosj, antpb, azaozz.
Fixes #49280.
Built from https://develop.svn.wordpress.org/trunk@47259


git-svn-id: https://core.svn.wordpress.org/trunk@47059 1a063a9b-81f0-0310-95a4-ce76da25c4cd