Merge pull request #10301 from manadart/2.5-write-leader-settings-guard

#10301

## Description of change

This is a reversion of a single commit from #9353.

The commit logs a warning instead of returning an error when `LeaderSettingsChanged` is called by a non-leader unit.

That commit should not have been retained in the patch, which ultimately focussed on re-checking leader status when preparing to fire the _leader-elected_ hook.

## QA steps

- Bootstrap.
- `juju deploy ubuntu -n 2`
- `juju run --unit <whichever is not the leader> leader-set`.
- Result should be: `ERROR cannot write leadership settings: cannot write settings: not the leader`.

## Documentation changes

None.

## Bug reference

Original: https://bugs.launchpad.net/juju-core/+bug/1723184

worker/uniter/runner/context/leader.go

@@ -74,10 +74,6 @@ func (ctx *leadershipContext) WriteLeaderSettings(settings map[string]string) er
 	// `apiserver/leadership.LeadershipSettingsAccessor.Merge`, and as of
 	// 2015-02-19 it's better to stay eager.
 	err := ctx.ensureLeader()
-	if err == errIsMinion {
-		logger.Warningf("skipping write settings; not the leader")
-		return nil
-	}
 	if err == nil {
 		// Clear local settings; if we need them again we should use the values
 		// as merged by the server. But we don't need to get them again right now;

worker/uniter/runner/context/leader_test.go

@@ -201,17 +201,16 @@ func (s *LeaderSuite) TestWriteLeaderSettingsMinion(c *gc.C) {
 	s.CheckCalls(c, []testing.StubCall{{
 		FuncName: "ClaimLeader",
 	}}, func() {
-		// We are not the leader.
+		// The first call fails...
 		s.tracker.results = []StubTicket{false}
 		err := s.context.WriteLeaderSettings(map[string]string{"blah": "blah"})
-		// No error, no call to Merge.
-		c.Check(err, jc.ErrorIsNil)
+		c.Check(err, gc.ErrorMatches, "cannot write settings: not the leader")
 	})
 
 	s.CheckCalls(c, nil, func() {
-		// ctx.isMinion is now true. No call to claim leader.
+		// The second doesn't even try.
 		err := s.context.WriteLeaderSettings(map[string]string{"blah": "blah"})
-		c.Check(err, jc.ErrorIsNil)
+		c.Check(err, gc.ErrorMatches, "cannot write settings: not the leader")
 	})
 }