Fix --preset-list argument and enhance error output

[hwwhww]

This PR fixes two issues in testgen:

Issue 1

• --preset-list flag was not implemented properly. The parameter presets was unused.

Fix

• Add if len(presets) != 0 and test_case.preset_name not in presets: continue to filter the presets
• Update README.md

---

Issue 2

• IOError was not logged properly. It was caught but went to sys.exit() immediately.

Fix

• Write exception to log file before sys.exit()