Say which input the image processor refuses before decoding

README.md, the doc comment of ErrUnsupportedInputFormat and the TODO.md
entry say the 415 is for input whose first bytes libvips reads as
another format, and no longer read as refusing a damaged image in a
source format, which is decoded and answers 500. The doc comment of
TestImageProcessor_OrigOfUnnamedFormatRefused says TIFF is refused
before it is decoded.

Model: opus-5-5

internal/imageprocessor/orig_format_internal_test.go

@@ -8,9 +8,9 @@ import (
 	"github.com/davidbyttow/govips/v2/vips"
 )
 
-// TestImageProcessor_OrigOfUnnamedFormatRefused verifies that orig of an
-// input in a format the processor decodes but cannot name, here TIFF, is
-// an error, and not the image saved as JPEG.
+// TestImageProcessor_OrigOfUnnamedFormatRefused verifies that orig of a TIFF
+// input, which is not a source format, is refused with
+// ErrUnsupportedInputFormat before it is decoded, and not saved as JPEG.
 func TestImageProcessor_OrigOfUnnamedFormatRefused(t *testing.T) {
 	t.Parallel()