graft: suggest the correct tool to continue (not graft)

Add test coverage for graft --continue without starting.
Suggest committing (or whatever the current activity is), via
wrongtooltocontinue which uses howtocontinue.

mercurial/commands.py

@@ -3933,7 +3933,7 b' def _dograft(ui, repo, *revs, **opts):'
         except IOError as inst:
             if inst.errno != errno.ENOENT:
                 raise
-            raise error.Abort(_("no graft state found, can't continue"))
+            cmdutil.wrongtooltocontinue(repo, _('graft'))
     else:
         cmdutil.checkunfinished(repo)
         cmdutil.bailifchanged(repo)

tests/test-graft.t

@@ -46,6 +46,13 b' Create a repo with some stuff in it:'
   |
   o  [e-mail]: 0
   
+Can't continue without starting:
+
+  $ hg rm -q e
+  $ hg graft --continue
+  abort: no graft in progress
+  [255]
+  $ hg revert -r . -q e
 
 Need to specify a rev: