[lldb] Fix assertions caused by un-checked errors in ScriptedProcess

This patch should fix some assertion that started getting hit after f22d82ce.

That commit changed the scripted object plugin creation to use
`llvm::Expected<T>` as a return type to enforce error handling, however
I forgot to handle the error which caused the assert.

The interesting  part about this, is that since that assert was triggered
in the ScriptedProcess constructor (where the `llvm::Error` wasn't
handled), that impacted every test that launched any kind of process,
since the process plugin manager would eventually also iterate over the
`ScriptedProcess::Create` factory method.

This patch should fix the assertions by handling the errors.

Signed-off-by: Med Ismail Bennani <ismail@bennani.ma>