Hcl Function Panics

startswith, endswith, strcontains, and run_cmd no longer panic on malformed calls

Calling startswith, endswith, or strcontains with the wrong number of arguments (for example a single argument instead of two) crashed Terragrunt instead of reporting a configuration error. Calling run_cmd with only option flags and no command (for example run_cmd("--terragrunt-quiet")) crashed the same way.

These calls now return a clear error: a wrong-number-of-parameters error for the string functions, and an empty-command error for run_cmd.