Ran across a few more issues while fixing error handling issues. So I'll bundle those fixes into this ticket as well:

FacetStream: was throwing NPE with zero facet results. TODO: add a test case for this. This effects the SQLHandler operating in facet aggregationMode.

GatherNodesStream: explain was breaking on the admin stream console, due to issues with toExpression.

CloudSolrStream: was throwing NPE on missing params rather then throwing proper exception. TODO: add test case for this.

GraphHandler: did not have the SelectStream registered.

Overall though the changes made in SOLR-8467 are looking good and the new admin stream console is working well.

Patch with fixes and tests. Since these are low risk fixes which really help usability I'd like to get this committed and back ported for Solr 6.1

Commit 97fd7988ae07adc49b03c9663b13b2d3bac3f4f9 in lucene-solr's branch refs/heads/master from jbernste
[ https://git-wip-us.apache.org/repos/asf?p=lucene-solr.git;h=97fd798 ]

SOLR-9128: Fix error handling issues in Streaming classes

Commit 1c7777cc558c2fec0a7c145e2b2194790ff2ad6a in lucene-solr's branch refs/heads/master from jbernste
[ https://git-wip-us.apache.org/repos/asf?p=lucene-solr.git;h=1c7777c ]

SOLR-9128: Update CHANGES.txt

Commit cb38b142051281ee740ae90c71aff4a7e933f452 in lucene-solr's branch refs/heads/branch_6x from jbernste
[ https://git-wip-us.apache.org/repos/asf?p=lucene-solr.git;h=cb38b14 ]

SOLR-9128: Fix error handling issues in Streaming classes

Commit 73b71b16d2911e413853561e2ee78e9a075e3451 in lucene-solr's branch refs/heads/branch_6x from jbernste
[ https://git-wip-us.apache.org/repos/asf?p=lucene-solr.git;h=73b71b1 ]

SOLR-9128: Update CHANGES.txt

Conflicts:
solr/CHANGES.txt

Not backporting to 6.0.1 because the Streaming Expression Explanation code (SOLR-9009) won't be backported to branch_6_0.