Incorrect error on db.shutDownServer() of 1 member replica set

Running db.shutdownServer() against a one member replica set fails with an error of "No electable secondaries caught up as of" with a timestamp. This makes no sense for a one member replica set.

We should either

1. Allow this command to execute without error.
2. Require {force: true} (as we do now) but provide a user-friendly message when run without and an example of the command to run.

   > use admin

   > db.shutdownServer()

   2015-03-06T16:21:44.474-0500 I NETWORK trying reconnect to 127.0.0.1:27017 (127.0.0.1) failed

   2015-03-06T16:21:44.475-0500 I NETWORK reconnect 127.0.0.1:27017 (127.0.0.1) ok

   assert failed : unexpected error: Error: shutdownServer failed: No electable secondaries caught up as of 2015-03-06T16:21:44.475-0500

   Error: assert failed : unexpected error: Error: shutdownServer failed: No electable secondaries caught up as of 2015-03-06T16:21:44.475-0500

   at Error (<anonymous>)

   at doassert (src/mongo/shell/assert.js:11:14)

   at assert (src/mongo/shell/assert.js:20:5)

   at DB.shutdownServer (src/mongo/shell/db.js:212:9)

   at (shell):1:4

   2015-03-06T16:21:44.476-0500 E QUERY Error: assert failed : unexpected error: Error: shutdownServer failed: No electable secondaries caught up as of 2015-03-06T16:21:44.475-0500

   at Error (<anonymous>)

   at doassert (src/mongo/shell/assert.js:11:14)

   at assert (src/mongo/shell/assert.js:20:5)

   at DB.shutdownServer (src/mongo/shell/db.js:212:9)

   at (shell):1:4 at src/mongo/shell/assert.js:13