Raise error for invalidly-formatted email addresses.

A message's `from_email` and each address in its `to`, `cc`, and `bcc` lists must contain exactly one email address. Previous code would silently ignore additional addresses, leading to unusual behavior. Now, raises new `AnymailInvalidAddress` exception.

Example: `from_email='Widgets, Inc. <widgets@example.com>'` is invalid: it needs double-quotes around the "Widgets, Inc." display-name portion. In earlier versions, this probably would have sent the message from something like "From: Widgets <@localhost>". Now, it will raise an exception.

**Potentially-breaking change:** If your code is using an unquoted display-name containing a comma in an email address, it will now raise an error. In earlier versions, this may have appeared to succeed, but was almost certainly not doing what you intended.

Fixes #44.

docs/sending/exceptions.rst

@@ -36,6 +36,26 @@ Exceptions
     your ESP's dashboard. See :ref:`troubleshooting`.)
 
 
+.. exception:: AnymailInvalidAddress
+
+    .. versionadded:: 0.7
+
+    The send call will raise a :exc:`!AnymailInvalidAddress` error if you
+    attempt to send a message with invalidly-formatted email addresses in
+    the :attr:`from_email` or recipient lists.
+
+    One source of this error can be using a display-name ("real name") containing
+    commas or parentheses. Per :rfc:`5322`, you should use double quotes around
+    the display-name portion of an email address:
+
+    .. code-block:: python
+
+        # won't work:
+        send_mail(from_email='Widgets, Inc. <widgets@example.com>', ...)
+        # must use double quotes around display-name containing comma:
+        send_mail(from_email='"Widgets, Inc." <widgets@example.com>', ...)
+
+
 .. exception:: AnymailSerializationError
 
     The send call will raise a :exc:`!AnymailSerializationError`