Synfig Animation Studio

When selecting the canvas and filename parameter in the parameters panel,
an error message is printed on stderr. It reads "error: EXCEPTION: bad
type: Bad type for curves". An examination of the code
(src/gtkmm/widget_curves.cpp, lines 222 and 397), reveals however that
nothing is wrong, and that this condition is subsequently ignored.

The problem originate from the fact that neither filenames nor canvasses
can be drawn as a curve, so they should be (and are) ignored by the curves
widget. The method of ignoring them, i.e. the exception thrown and caught,
results however also in the printing of an error message, and this is
wrong.

Attached is a patch that consistutes probably the easiest way to avoid this
problem. Another solution would be to introduce a new exception, that
doesn't also constitute an error, e.g. type_not_supported, or move the
condition in the if-statement to the CurveStruct class, to enhance
encapsulation.