Add `image_not_processable` localization #199

Job	Run time
minitest (2.6, rails_6_0, vips)	50s
minitest (2.6, rails_6_0, mini_magick)	2m 7s
minitest (2.6, rails_6_1, vips)	50s
minitest (2.6, rails_6_1, mini_magick)	58s
minitest (2.7, rails_6_0, vips)	49s
minitest (2.7, rails_6_0, mini_magick)	52s
minitest (2.7, rails_6_1, vips)	56s
minitest (2.7, rails_6_1, mini_magick)	1m 12s
minitest (2.7, rails_7_0, vips)	1m 0s
minitest (2.7, rails_7_0, mini_magick)	1m 7s
minitest (2.7, rails_next, vips)	1m 6s
minitest (2.7, rails_next, mini_magick)	1m 21s
minitest (3.0, rails_6_1, vips)	1m 9s
minitest (3.0, rails_6_1, mini_magick)	39s
minitest (3.0, rails_7_0, vips)	1m 29s
minitest (3.0, rails_7_0, mini_magick)	38s
minitest (3.0, rails_next, vips)	43s
minitest (3.0, rails_next, mini_magick)	34s
minitest (3.1, rails_7_0, vips)	35s
minitest (3.1, rails_7_0, mini_magick)	1m 4s
minitest (3.1, rails_next, vips)	45s
minitest (3.1, rails_next, mini_magick)	1m 36s
minitest (3.2, rails_7_0, vips)	40s
minitest (3.2, rails_7_0, mini_magick)	35s
minitest (3.2, rails_next, vips)	41s
minitest (3.2, rails_next, mini_magick)	45s
	25m 1s

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