subrepo: chain the original exception to SubrepoAbort...
subrepo: chain the original exception to SubrepoAbort
The tracebacks in subrepos are truncated at the point where the original
exception is caught and SubrepoAbort is raised in its place since 9e3910db4e78.
That hides the most relevant subrepo methods when an error occurs. Python 2.x
doesn't support chaining exceptions, so it is manually done here for manual
printing later.
