- error messages when writing PNM images were always
'unable to write pnm image', more useful messages are now reported.
- convert t/t103raw.t to Test::More
+- reading a raw image no longer exits on a short read or read error,
+ and returns an appropriate error message in $im->errstr
+- write failures when writing a raw image now return a useful
+ message in $im->errstr
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