There are a few places in Shadow that call panic() if user input is invalid, which prints a backtrace. Instead we should exit gracefully.

Known issues:

• panic when network graph file doesn't exist — Crash when network graph file does not exist #1459
• panic when the managed process binary doesn't exist - Provide a clearer error message when process binary isn't found #1522
• panic when data directory already exists
• panic when template directory does not exist
• abort (no message or backtrace) when the managed process path exists but is not executable
• panic when an out-of-range network_node_id is given (networkgraph_nodeBandwidthDownBits panicked at 'called Option::unwrap() on a None value')
• panic when the configuration has no hosts (config_getWorkers panicked at 'called Option::unwrap() on a None value')
• panic when the network has an edge with a latency of 0 ("Message: need nonzero latency to set buffer sizes, source=1 dest=2 send=0 recv=0")
• deadlock when a process specifies a stop_time where stop_time <= start_time
• panic when a host IP address is specified as an IPv6 address