Reduce discrepancies with respect to argparse

[mauvilsa]

jsonargparse has a few deliberate deviations with respect to argparse. However, there are some differences which are not needed and it is preferable to remove them to stay more aligned with argparse.

• [x] Support exit_on_error (#231)
• [x] When parsing fails, use ArgumentError exception (#231)
• [ ] Handle default type conversion with ArgumentTypeError exception
• [ ] Run unit tests from argparse to guarantee equivalent behavior
• [ ] Document deliberate deviations from argparse