[lldb] Don't print IRForTarget errors directly to the console

Summary:

When we get an error back from IRForTarget we directly print that error to the
debugger output stream instead of putting it in the result object. The result
object only gets a vague "The expression could not be prepared to run in the
target" error message that doesn't actually tell the user what went wrong.

This patch just puts the IRForTarget errors into the status object that is
returned to the caller instead of directly printing it to the debugger. Also
updates one test that now can actually check for the error message it is
supposed to check for (instead of the default error which is all we had before).

Reviewers: JDevlieghere

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D81654