Do not call TTree cache after an exception happens

[Dr15Jones]

Since TTree is not exception safe, we must avoid calling TTree methods after we have had an exception thrown through TTree. Normally this happens when one thread throws an exception while another thread is waiting to access the same TTree.
This should avoid crashes seen in the Tier 0.

[cmsbuild]

A new Pull Request was created by @Dr15Jones (Chris Jones) for CMSSW_7_5_X.

It involves the following packages:

IOPool/Input

@cmsbuild, @smuzaffar, @Dr15Jones, @davidlange6 can you please review it and eventually sign? Thanks.
@wddgit, @wmtan this is something you requested to watch as well.
@slava77, @Degano, @smuzaffar you are the release manager for this.

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[Dr15Jones]

please test

[Dr15Jones]

+1

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/10126/console

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_5_X IBs after it passes the integration tests and once validation in the development release cycle CMSSW_7_6_X is complete. This pull request requires discussion in the ORP meeting before it's merged. @slava77, @davidlange6, @Degano, @smuzaffar

[Dr15Jones]

@davidlange6 @slava77 this should avoid the segmentation fault seen in the Tier 0 which happens sometimes when there is an xrootd error.

[cmsbuild]

-1
Tested at: 0594e7a
When I ran the RelVals I found an error in the following worklfows:
50202.0 step2

runTheMatrix-results/50202.0_TTbar_13+TTbar_13+DIGIUP15_PU50+RECOUP15_PU50+HARVESTUP15_PU50/step2_TTbar_13+TTbar_13+DIGIUP15_PU50+RECOUP15_PU50+HARVESTUP15_PU50.log
----- Begin Fatal Exception 03-Dec-2015 23:35:43 CET-----------------------
An exception of category 'Configuration' occurred while
   [0] Constructing the EventProcessor
   [1] Constructing module: class=MixingModule label='mix'
Exception Message:
RootEmbeddedFileSequence no input files specified for secondary input source.
----- End Fatal Exception -------------------------------------------------

1001.0 step1

DAS Error

1003.0 step1

DAS Error

you can see the results of the tests here:
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12663/10126/summary.html