tsweb: add a Handler type.

Handler is like http.Handler, but returns errors. ErrHandler
converts back to an http.Handler, with added error handling
and logging.

Signed-off-by: David Anderson <dave@natulte.net>

tsweb/log.go

@@ -43,7 +43,9 @@ func (m Msg) String() string {
 	return strings.TrimRight(buf.String(), "\n")
 }
 
-// HTTPError is an error with embedded HTTP response information.
+// HTTPError is an error with embedded HTTP response information. When
+// received by an ErrHandler, the Code and Msg are sent to the client,
+// while Err is logged on the server.
 type HTTPError struct {
 	Code int
 	Msg  string // sent to the end-user