Better error message.

theano/gof/fg.py

@@ -4,8 +4,10 @@ fg.py: fg stands for FunctionGraph
 Contains the FunctionGraph class and exception
 types that it can raise
 """
+import StringIO
 import sys
 import time
+import traceback
 
 import theano
 from theano.gof import graph
@@ -328,18 +330,29 @@ class FunctionGraph(utils.object2):
                             #if there is no path then r isn't really a graph input so we shouldn't be running error
                             #handler code in the first place
                             assert path is not None
-
-                            raise MissingInputError((
+                            tr = getattr(r.tag, 'trace', None)
+                            detailed_err_msg = ""
+                            if tr:
+                                sio = StringIO.StringIO()
+                                traceback.print_list(tr, sio)
+                                tr = sio.getvalue()
+                                detailed_err_msg += "\nBacktrace when the variable is created:\n"
+                                detailed_err_msg += str(tr)
+
+                            raise MissingInputError(
                                 'A variable that is an input to the graph was '
                                 'neither provided as an input to the function '
                                 'nor given a value. A chain of variables '
                                 'leading from this input to an output is %s. '
-                                'This chain may not be unique' % str(path)))
+                                'This chain may not be unique' % str(path) +
+                                detailed_err_msg)
 
                         #Standard error message
                         raise MissingInputError((
                             "An input of the graph, used to compute %s, "
-                            "was not provided and not given a value"
+                            "was not provided and not given a value."
+                            "Use the Theano flag exception_verbosity='high',"
+                            "for more information on this error."
                             % str(node)),
                             r)