Python: fix stack trace exposure query.

[markshannon]

Avoid cross-talk between sources by more tightly specifying the type of the source.

Fixes a recent regression, so no change note needed.

We will want to prevent this in general for the next release by adding some sort of configuration, so the query would become something like

class InformationExposureConfiguration extends TaintTracking::Configuration {
    predicate isSource(TaintTracking::Source source) { source.getNode() instanceof ErrorInfoSource }
    predicate isSink(TaintTracking::Sink sink) { sink.getNode() instanceof ErrorInfoSink }
}
from InformationExposureConfiguration config, TaintedPathSource src, TaintedPathSink sink
where config.hasFlowPath(src, sink)
select sink.getNode(), src, sink, "$@ may be exposed to an external user", src.getSource(), "Error information"

However, that is probably too intrusive a change this close to the release.