added the possibility to register a transformation context specific error

added the possibility to register a transformation context specific error

* libxslt/xsltInternals.h libxslt/xsltutils.[ch]: added the
  possibility to register a transformation context specific
  error handler, with xsltSetTransformErrorFunc() and provided
  a new routine xsltTransformError() to handle contextual errors,
  this should fix #94435
* libxslt/*.c: modified all the code to use the context specific
  error handling, as a result xsltPrintErrorContext() is not called
  anymore except internally from xsltTransformError()
Daniel