Refine the template processing in QuantiPhyError.render().

doc/user.rst

@@ -2094,4 +2094,18 @@ a format string that can access both the unnamed and named arguments:
     ...     q = Quantity('tweed')
     ... except InvalidNumber as e:
     ...     print(e.render('{}: no es un número valido.'))
+    ... except UnknownScaleFactor as e:
+    ...     print(e.render('factor de escala desconocido.'))
+    ... except UnknownConversion as e:
+    ...     if 'direction' in e.kwargs:
+    ...         direction = e.kwargs['direction']
+    ...         if direction == 'to':
+    ...             template = 'incapaz de convertir a {}'
+    ...         else:  # direction must be 'from'
+    ...             template = 'incapaz de convertir de {}'
+    ...     else:
+    ...         template = 'incapaz de convertir entre {} y {}'
+    ...     print(e.render(template))
+    ... except InvalidRecognizer as e:
+    ...     print(e.render("el reconocedor no contiene la clave 'val'"))
     tweed: no es un número valido.

quantiphy.py

@@ -115,7 +115,7 @@ def render(self, template=None):
                 if msg == template and self.args:
                     break
                 return msg
-            except IndexError:
+            except (IndexError, KeyError):
                 continue
         culprits = ', '.join(str(a) for a in self.args)
         return '{}: {}'.format(culprits, template)
@@ -174,8 +174,8 @@ class UnknownConversion(QuantiPhyError, KeyError):
     conversion was requested and there is no corresponding unit converter.
     """
     _template = (
-        "unable to convert between '{}' and '{}'.",
         "unable to convert {direction} '{}'.",
+        "unable to convert between '{}' and '{}'.",
     )