add MissingSubKeyError for config loading. update main.py for --config flag

src/pg_rad/exceptions/exceptions.py

@@ -12,3 +12,14 @@ class InvalidCSVError(DataLoadError):
 
 class OutOfBoundsError(Exception):
     """Raised when an object is attempted to be placed out of bounds."""
+
+
+class MissingNestedKeyError(Exception):
+    """Raised when a nested key is missing in the config."""
+    def __init__(self, key, subkey=None):
+        if subkey:
+            self.message = f"Missing key in {key}: {subkey}"
+        else:
+            self.message = f"Missing key: {key}"
+
+        super().__init__(self.message)