Return the internal error instead of the ACME error

For ACME errors, return the internal error string instead of the
ACME one on the "Error() string" function. This way the logs will
have more information about the cause of an error.

Fixes #1057

acme/errors.go

@@ -337,7 +337,10 @@ func (e *Error) StatusCode() int {
 
 // Error allows AError to implement the error interface.
 func (e *Error) Error() string {
-	return e.Detail
+	if e.Err == nil {
+		return e.Detail
+	}
+	return e.Err.Error()
 }
 
 // Cause returns the internal error and implements the Causer interface.