Add context in case of exception from BranchDescription

When initializing a BranchDescription from a ROOT dictionary, if
there is an exception thrown we now add to the context information
which branch had the problem.
Do not throw an exception when ROOT reports a missing dictionary problem

In the case where ROOT prints a message about a missing collection
proxy we no longer throw an exception. This was needed to handle the
case of reading an old file using a release for which a class in that
file no longer exists. The framework doesn't need the exception since
it has other ways of detecting the missing dictionary and then take
appropriate action.
Merge pull request #4931 from Dr15Jones/ignoreMessageCausedByMissingDict

Do not throw an exception when ROOT reports a missing dictionary problem.