templater: give better error message for invalid engine type

Before, KeyError was caught at changeset_templater._show(), which said "no
key named '%s'" as it was intended to catch the KeyError of unknown map key.
Instead, we should catch KeyError explicitly for better error indication.

For those who don't know what the template engine is (read "everyone"), it is
hidden extension feature that allows switching template syntax in map file.
See d8c5a7f25a40 for details.

mercurial/templater.py

@@ -1011,8 +1011,11 b' class templater(object):'
     def __call__(self, t, **mapping):
         ttype = t in self.map and self.map[t][0] or 'default'
         if ttype not in self.ecache:
-            self.ecache[ttype] = engines[ttype](self.load,
-                                                 self.filters, self.defaults)
+            try:
+                ecls = engines[ttype]
+            except KeyError:
+                raise error.Abort(_('invalid template engine: %s') % ttype)
+            self.ecache[ttype] = ecls(self.load, self.filters, self.defaults)
         proc = self.ecache[ttype]
 
         stream = proc.process(t, mapping)

tests/test-template-engine.t

@@ -44,4 +44,11 b''
   0 97e5f848f0936960273bbf75be6388cd0350a32b -1 0000000000000000000000000000000000000000
   -1 0000000000000000000000000000000000000000 -1 0000000000000000000000000000000000000000
 
+invalid engine type:
+
+  $ echo 'changeset = unknown:changeset.txt' > unknownenginemap
+  $ hg log --style=./unknownenginemap
+  abort: invalid template engine: unknown
+  [255]
+
   $ cd ..