Check if output directory is valid and writable before starting SLOTHY

[hanno-becker]

When passing an invalid output directory (e.g. one which doesn't exist, or which can't be written to), the user will only find out when the SLOTHY optimization completes, which for complicated examples can take hours. When this happens, the time and cost of optimization is currently wasted, because SLOTHY fails without having stored the result anywhere. Additionally, it leaves behind a frustrated user.

• [ ] Check if the output directory exists and is writable before starting SLOTHY optimization
• [ ] Optional: Catch any error while storing the optimized code, and dump it to a temporary file or the std output in this case.