This replaces the current use of LLDB's own StringConvert with LLVM's to_integer which
has a less error-prone API and doesn't use special 'error values' to designate parsing problems.
Where needed I also added missing error handling code that prints a parsing error instead of continuing
with the error value returned from StringConvert (which either gave a cryptic error message or just took
the error value performed an incorrect action with it. For example, frame recognizer delete -1
just deleted the frame recognizer at index 0).