[docs] Add more Error documentation to the Programmer's Manual.

This patch updates some of the existing Error examples, expands on the
documentation for handleErrors, and includes new sections that cover
a number of helpful utilities and common error usage idioms.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285122 91177308-0d34-0410-b5e6-96231b3b80d8