mu4e: better error for non-supported header fields

Some fields (eg. :attachments and :user-agent) require a full message
and are not supported in headers-mode. Document this and give a clearer
error message when they are added to `mu4e-headers-fields'.

Fixes issue #933.

mu4e/mu4e-headers.el

@@ -851,7 +851,8 @@ after the end of the search results."
 	  (let* ((field (car item)) (width (cdr item))
 		  (info (cdr (assoc field
 			       (append mu4e-header-info mu4e-header-info-custom))))
-		  (sortable (plist-get info :sortable))
+		  (require-full (plist-get info :require-full))
+		  (sortable (plist-get info :sortable))		  
 		  ;; if sortable, it is either t (when field is sortable itself)
 		  ;; or a symbol (if another field is used for sorting)
 		  (sortfield (when sortable (if (booleanp sortable) field sortable)))
@@ -862,6 +863,8 @@ after the end of the search results."
 		      (if (eq mu4e-headers-sort-direction 'descending) downarrow uparrow)))
 		  (name (concat (plist-get info :shortname) arrow))
 		  (map (make-sparse-keymap)))
+	    (when require-full
+	      (mu4e-error "Field %S is not supported in mu4e-headers-mode" field))
 	    (when sortable
 	      (define-key map [header-line mouse-1]
 		(lambda (&optional e)

mu4e/mu4e-vars.el

@@ -689,6 +689,7 @@ mu4e-compose-mode."
        ( :name "Attachments"
 	 :shortname "Atts"
 	 :help "Message attachments"
+	 :require-full t
 	 :sortable nil))
      (:bcc .
        ( :name "Bcc"
@@ -749,11 +750,13 @@ mu4e-compose-mode."
        ( :name "Signature"
 	 :shortname "Sgn"
 	 :help "Check for the cryptographic signature"
+	 :require-full t
 	 :sortable nil))
      (:decryption .
        ( :name "Decryption"
 	 :shortname "Dec"
 	 :help "Check the cryptographic decryption status"
+	 :require-full t
 	 :sortable nil))
      (:size .
        ( :name "Size"
@@ -784,6 +787,7 @@ mu4e-compose-mode."
        ( :name "User-Agent"
 	 :shortname "UA"
 	 :help "Program used for writing this message"
+	 :require-full t
 	 :sortable t)))
   "An alist of all possible header fields and information about them.
 This is used in the user-interface (the column headers in the header list, and
@@ -799,7 +803,12 @@ sort by this field.  This can be either a boolean (nil or t), or a
 symbol for /another/ field. For example, the `:human-date' field
 uses `:date' for that.
 
-Note, `:sortable' does not work for custom header fields.")
+Fields with which have the property `:require-full' set to
+non-nil require a full message; in practice this means that you
+cannot use such fieds as part of `mu4e-headers-fields', but only
+in `mu4e-view-fields.'
+
+Note, `:sortable' is not supported for custom header fields.")
 
 
 (defvar mu4e-header-info-custom