[lldb/Expression] Improve interpreter error message with a non-running target

When trying to interpret an expression with a function call, if the
process hasn't been launched, the expression fails to be interpreted
and the user gets the following  error message:

```error: Can't run the expression locally```

This message doesn't explain why the expression failed to be
interpreted, that's why this patch improves the error message that is
displayed when trying to run an expression while no process is running.

rdar://11991708

Differential Revision: https://reviews.llvm.org/D72510



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>