revsets: more informative syntax error message

I came across a case where an internal command was using a revset that I didn't
immediately pass in and it was difficult to debug what was going wrong with the
revset. This prints out the revset and informs the user that the error is with
a rebset so it should be more obvious what and where the error is.

mercurial/revset.py

@@ -241,7 +241,8 b' def tokenize(program, lookup=None, symin'
                 yield ('symbol', sym, s)
             pos -= 1
         else:
-            raise error.ParseError(_("syntax error"), pos)
+            raise error.ParseError(_("syntax error in revset '%s'") %
+                                   program, pos)
         pos += 1
     yield ('end', None, pos)
 

tests/test-revset.t

@@ -1535,7 +1535,7 b' test in problematic encoding'
 
 test error message of bad revset
   $ hg log -r 'foo\\'
-  hg: parse error at 3: syntax error
+  hg: parse error at 3: syntax error in revset 'foo\\'
   [255]
 
   $ cd ..