Set error handler at constructor to prevent calling it several times

Event handler onKernelRequest can be called several times: for main request
and subrequests. In that case property "previousErrorHandler" becomes
overriden with RollbarListener's handler that causing calling itself
endlessly.

Setting error handler at the constructor fixes it.
Merge pull request #5 from tutitututu/fix-subrequest-error-handler

Prevent setting error handler on subrequests