[Python] FIx Contextualizer validation to arguments (#168)

Closes #164

---------

Co-authored-by: Will Jones <willjones127@gmail.com>

python/lancedb/exceptions.py

@@ -0,0 +1,16 @@
+"""Custom exception handling"""
+
+class MissingValueError(ValueError):
+    """Exception raised when a required value is missing."""
+    pass
+
+class MissingColumnError(KeyError):
+    """
+    Exception raised when a column name specified is not in 
+    the  DataFrame object
+    """
+    def __init__(self, column_name):
+        self.column_name = column_name
+
+    def __str__(self):
+        return f"Error: Column '{self.column_name}' does not exist in the DataFrame object"