Update README.md to enhance MicromailClient documentation and add exceptions section; modify error handling in write method to raise NoSocketError

micromail.py

@@ -110,7 +110,7 @@ class MicromailClient:
 
     def write(self, data: str | bytes) -> None:
         if self.socket is None:
-            raise Error("No socket to write to.")
+            raise NoSocketError
 
         if isinstance(data, str):
             data = data.encode()