more granular error messages

[MartijnR]

in both preview and webform, the errors:

For webform
"An Error Was Encountered
Form not reachable (or an error occurred during transformation)."

For Preview
" An error occurred trying to obtain or transform the form."

has at least 3 possible causes

a) the xform wasn't reachable
b) the xform wasn't valid (just last week, I had someone try to use a form with xml version="2.0", which is I think in a very preliminary stage so he probably made a mistake)
c) the form id is non-existing on the /formList (but this error seems unlikely if they accessed if via a link in the SurveyCTO interface)

I should make a distinction between the error messages of these 3 causes.