Handle long-loading indication for conversations better

Moving the loading placeholder from ConversationListBox to
ConversationEmail allows a more fine-grained indication of what is
happening - only show the loading indicator when the remote actually
needs to get hit, display the email's details and load the rest of the
conversation while waiting for the remote body load. Also lets us pass
errors loading the initial email locally all the way up to the
controller.

ui/conversation-message.ui

@@ -595,6 +595,7 @@
             </child>
             <child>
               <object class="GtkOverlay">
+                <property name="height_request">6</property>
                 <property name="visible">True</property>
                 <property name="can_focus">False</property>
                 <child>