Release v2.4.2

• bugfix: PSFContents used BaseObjList.get() to filter atoms by segtype when building the topology-active atom list for ring_check; get() returns None for zero matches and a bare object for exactly one match, causing a TypeError on len(); replaced with filter(), which always returns a BaseObjList
• new feature: custom exception hierarchy (PestiferError, PestiferBuildError) replaces bare RuntimeError/ValueError/Exception raises throughout the task and molecule layers; the CLI top-level handler catches any PestiferError and logs a clean error message with sys.exit(1) rather than printing a Python traceback; ring_check with delete: none now raises PestiferBuildError instead of silently logging