refactor: enhance code structure and add Terms of Service handling for API interactions

gemini_key_manager/exceptions.py

@@ -0,0 +1,10 @@
+"""
+Custom exceptions for the Gemini Key Management script.
+"""
+
+class TermsOfServiceNotAcceptedError(Exception):
+    """Raised when the Terms of Service for a required API have not been accepted."""
+    def __init__(self, message, url):
+        self.message = message
+        self.url = url
+        super().__init__(self.message)