Better error reporting when a variable can't be
read during materialization.  First of all, report
if we can't read the data for some reason.  Second,
consult the ValueObject's error and report that if
there's some problem.

<rdar://problem/16074201>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@202552 91177308-0d34-0410-b5e6-96231b3b80d8