Change local variable name.
Give more evidence to hint for more debug information.
Better backtrace storage.

Raise the backtrace limit to caught where v1 + v2 is created in user script.
Remove the add_tag_trace from the bt.
When a thunk fail, print the back-trace of the node creation.
when a thunk fail, always print the node.inputs types.
Put all hints at the end of the error message.

this is more clear and allow to see all of them at once.
Merge pull request #1736 from nouiz/err_msg

[MRG]Better error message when a thunk fail: print user code line when node was created