Add error-level warning from original impl

Author: searls

src/log/ensure-promise.js

@@ -1,12 +1,22 @@
 import config from '../config'
 import log from '../log'
 
+const MESSAGES = {
+  warn: `\
+no promise constructor is set, so this \`thenResolve\` or \`thenReject\` stubbing
+will fail if it's satisfied by an invocation on the test double. You can tell
+testdouble.js which promise constructor to use with \`td.config\`, like so:`,
+  error: `\
+no promise constructor is set (perhaps this runtime lacks a native Promise
+function?), which means this stubbing can't return a promise to your
+subject under test, resulting in this error. To resolve the issue, set
+a promise constructor with \`td.config\`, like this:`
+}
+
 export default function ensurePromise (level) {
   if (config().promiseConstructor == null) {
     log[level]('td.when', `\
-no promise constructor is set, so this \`thenResolve\` or \`thenReject\` stubbing
-will fail if it's satisfied by an invocation on the test double. You can tell
-testdouble.js which promise constructor to use with \`td.config\`, like so:
+${MESSAGES[level]}
 
   td.config({
     promiseConstructor: require('bluebird')

test/unit/log/ensure-promise.test.js

@@ -6,7 +6,7 @@ module.exports = {
 
     subject = require('../../../src/log/ensure-promise').default
   },
-  'config has no promise set': () => {
+  'config has no promise set (warn)': () => {
     td.when(config()).thenReturn({promiseConstructor: null})
 
     subject('warn')
@@ -36,9 +36,10 @@ testdouble.js which promise constructor to use with \`td.config\`, like so:
     subject('error')
 
     td.verify(log.error('td.when', `\
-no promise constructor is set, so this \`thenResolve\` or \`thenReject\` stubbing
-will fail if it's satisfied by an invocation on the test double. You can tell
-testdouble.js which promise constructor to use with \`td.config\`, like so:
+no promise constructor is set (perhaps this runtime lacks a native Promise
+function?), which means this stubbing can't return a promise to your
+subject under test, resulting in this error. To resolve the issue, set
+a promise constructor with \`td.config\`, like this:
 
   td.config({
     promiseConstructor: require('bluebird')

test/unit/share/create-promise.test.js

@@ -54,6 +54,6 @@ module.exports = {
 
       td.verify(rejecter('pants'))
       assert.equal(td.explain(resolver).callCount, 0)
-    },
+    }
   }
 }