Patch: Partial fix for bug 2368 (version 2)

This patch provides a partial fix for bug 2368.

The second image in the cited URL is CMY, and the libraries fail to
convert it. The resulting bitmap is null. It is rendered as a
pink rectangle. If the user tries to export it as a sprite,
riscos_bitmap_save() is called, which immediately attempts a null
dereference, causing a crash (when I tried it, it was a serious
crash that brought the whole machine down).

The simple fix is to test for a null bitmap and, if so, warn the
user and go no further.

This version has support for internationalisation of the error
message, and I've supplied messages in the three languages I can
muster.

Longer term of course we should enhance the libraries so that the
conversion does not fail.

Dave

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