[lldb] Improve error message when evaluating expression when not stopped

When trying to run an expression after a process has existed, you
currently are shown the following error message:

  (lldb) p strlen("")
  error: Can't make a function caller while the process is running

This error is wrong and pretty uninformative. After this patch, the
following error message is shown:

  (lldb) p strlen("")
  error: unable to evaluate expression while the process is exited: the
  process must be stopped because the expression might require
  allocating memory.

rdar://109731325

Differential revision: https://reviews.llvm.org/D151497