[docs] Add a pointer to ExitOnError to the discussion of handleErrors in the

[docs] Add a pointer to ExitOnError to the discussion of handleErrors in the
programmer's manual.

ExitOnError is often a better alternative to handleErrors for tool code. This
patch makes it easier to find the ExitOnError discussion when reading the
handleErrors section.

Thanks to Peter Collingbourne for the suggestion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286167 91177308-0d34-0410-b5e6-96231b3b80d8