Bug 707239 - Gimp destroys image file when exporting...

as .tif with jpeg compression

make unsensitive the JPEG radio button when the image
is indexed, similarly to Fax Group 3/4 compression
enabled only for monochrome images

move the file creation/truncation after checking for
invalid requests and return a GError in case of error

(cherry picked from commit 7e80e2e3bc77f17dac3fc0f712970a34215a6814)

Conflicts:
	plug-ins/common/file-tiff-save.c