more legible error message in config.get/set

Test added by @isaacs
Closes #4458

lib/npm.js

@@ -31,7 +31,15 @@ var EventEmitter = require("events").EventEmitter
   , chain = slide.chain
   , RegClient = require("npm-registry-client")
 
-npm.config = {loaded: false}
+npm.config = {
+  loaded: false,
+  get: function() {
+    throw new Error('npm.load() required')
+  },
+  set: function() {
+    throw new Error('npm.load() required')
+  }
+}
 
 // /usr/local is often a read-only fs, which is not
 // well handled by node or mkdirp.  Just double-check

test/tap/npm-api-not-loaded-error.js

@@ -0,0 +1,47 @@
+var test = require("tap").test
+var npm = require("../..")
+var path = require("path")
+var rimraf = require("rimraf")
+var npmrc = path.join(__dirname, "npmrc")
+var fs = require("fs")
+
+test("setup", function (t) {
+  fs.writeFileSync(npmrc, "foo = bar\n", "ascii")
+  t.end()
+})
+
+test("calling set/get on config pre-load should throw", function (t) {
+  var threw = true
+  try {
+    npm.config.get("foo")
+    threw = false
+  } catch (er) {
+    t.equal(er.message, "npm.load() required")
+  } finally {
+    t.ok(threw, "get before load should throw")
+  }
+
+  var threw = true
+  try {
+    npm.config.set("foo", "bar")
+    threw = false
+  } catch (er) {
+    t.equal(er.message, "npm.load() required")
+  } finally {
+    t.ok(threw, "set before load should throw")
+  }
+
+  npm.load({ userconfig: npmrc }, function (er) {
+    if (er)
+      throw er
+    t.equal(npm.config.get("foo"), "bar")
+    npm.config.set("foo", "baz")
+    t.equal(npm.config.get("foo"), "baz")
+    t.end()
+  })
+})
+
+test("cleanup", function (t) {
+  rimraf.sync(npmrc)
+  t.end()
+})