dfmc-environment-projects: build-project: add type to error-handler

`error-handler` is required to be a function since it's passed to
`project-condition-handler` via `with-project-location-handler`. (Took me a
while to track down given the backtrace doesn't help much when there's an error
in the error handler.)

sources/environment/dfmc/projects/projects.dylan

@@ -311,7 +311,8 @@ define sealed method build-project
           abort-on-all-warnings? = #f,
           abort-on-serious-warnings? = #f,
           warning-callback :: false-or(<function>),
-          progress-callback :: false-or(<function>), error-handler,
+          progress-callback :: false-or(<function>),
+          error-handler :: <function>,
           save-databases? = #f,
           process-subprojects? = #t,
           messages = #"external")