Catch encoding exception when performing instant preview on an uncodable char (bug...

Catch encoding exception when performing instant preview on an uncodable char (bug 6116).

* MathStream{.cpp, h}:
	- replace bool dryrun() by enum output that also knows whether the stream is for instant preview
* InsetMathHull.cpp:
	- tell the stream whether we use it for instant preview.
* MathString.cpp (write):
	- gracefully catch encoding exception for instant preview.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@30895 a592a061-630c-0410-9148-cb99ea01b6c8