templater: add symbol to error

This patch makes it easier to debug writing templater functions by
telling the user exactly what was sent to the templater.

mercurial/templater.py

@@ -111,7 +111,7 b' def compileexp(exp, context):'
 def getsymbol(exp):
     if exp[0] == 'symbol':
         return exp[1]
-    raise error.ParseError(_("expected a symbol"))
+    raise error.ParseError(_("expected a symbol, got '%s'") % exp[0])
 
 def getlist(x):
     if not x:

tests/test-command-template.t

@@ -1911,3 +1911,9 b' Test startswith'
   |
   o  line 1
      line 2
+
+Test bad template with better error message
+
+  $ hg log -Gv -R a --template '{desc|user()}'
+  hg: parse error: expected a symbol, got 'func'
+  [255]