msSaveImage() failures for GetCoverage don't result in error message

[warmerdam]

Steve,

I was doing a little WCS test, and accidentally ran with a WEB IMAGEPATH
set to a directory that didn't exist.  This resulted in the msSaveImage()
failing because it could not create the temporary file.  

An msSetError() call was made down in msSaveImageGDAL(), but the error was
never turned into an exception "up the stack". 

I *think* that this should likely be done in msWCSGetCoverage() shortly
after the msSaveImage() call if the returned status is not MS_SUCCESS.   Looking
at the code I notice that msDrawRasterLayerLow() returning an error also
does not result in any sort of exception.  Should it?

[fwarmerdam]

I have corrected both these by throwing a WCS exception if these errors occur.

Note that if msSaveImage() fails, the exception will have already been
improperly typed with the image result mime type but at that point there is
little we can do to fix the problem.