fix(retry): be more specific about when we retry

Retries are now attempted only for specific known errors. All other will
fail fast. Additionally, requests will be retried on 404s and
rate-limiting status codes 420 (for Twitter) and 429, along with the
original 408 and 500-level error codes. All other codes will continue to
fast-path to request failure.

One big effect of this is that requests are no longer retried on
ENOTFOUND -- that is, the getaddrinfo DNS failure error. When this
happens, the problem is almost certainly that the user is currently
offline. In combination with conditional requests and the cache rules,
this means that make-fetch-happen now has effectively an "offline mode",
where the `default` cache mode will only fail on ENOTFOUND if there is
no cache entry available.

As before, if we're using a stale locally-cached request due to an error
(again, such as ENOTFOUND), a `Warning: 111` header will be added to the
response, so you can check that field to figure out what
make-fetch-happen did.

index.js

@@ -8,6 +8,19 @@ const retry = require('promise-retry')
 let ssri
 const Stream = require('stream')
 
+const RETRY_ERRORS = [
+  'ECONNRESET', // remote socket closed on us
+  'ECONNREFUSED', // remote host refused to open connection
+  'EADDRINUSE', // failed to bind to a local port (proxy?)
+  'ETIMEDOUT' // someone in the transaction is WAY TOO SLOW
+  // Known codes we do NOT retry on:
+  // ENOTFOUND (getaddrinfo failure. Either bad hostname, or offline)
+]
+
+const RETRY_TYPES = [
+  'request-timeout'
+]
+
 // https://fetch.spec.whatwg.org/#http-network-or-cache-fetch
 module.exports = cachingFetch
 cachingFetch.defaults = function (_uri, _opts) {
@@ -196,7 +209,7 @@ function condFetch (uri, cachedRes, opts) {
     if (ctrl.match(/must-revalidate/i)) {
       throw err
     } else {
-      setWarning(cachedRes, 111, `Unexpected error: ${err.message}`)
+      setWarning(cachedRes, 111, `${err.code}: ${err.message}`)
       return cachedRes
     }
   })
@@ -280,8 +293,18 @@ function remoteFetch (uri, opts) {
             return res
           }
         })
-      } else if (res.status === 408 || res.status >= 500) {
-        // 408 === timeout
+      } else if (
+        // Retriable + rate-limiting status codes
+        // When hitting an API with rate-limiting features,
+        // be sure to set the `retry` settings according to
+        // documentation for that.
+        res.status === 404 || // Not Found ("subsequent requests permissible")
+        res.status === 408 || // Request Timeout
+        res.status === 420 || // Enhance Your Calm (usually Twitter rate-limit)
+        res.status === 429 || // Too Many Requests ("standard" rate-limiting)
+        // Assume server errors are momentary hiccups
+        res.status >= 500
+      ) {
         if (req.method === 'POST') {
           return res
         } else if (req.body instanceof Stream) {
@@ -293,14 +316,22 @@ function remoteFetch (uri, opts) {
         return res
       }
     }).catch(err => {
-      if (req.method !== 'POST') {
+      const code = err.code === 'EPROMISERETRY'
+      ? err.retried.code
+      : err.code
+      if (
+        req.method !== 'POST' && (
+          RETRY_ERRORS.indexOf(code) >= 0 ||
+          RETRY_TYPES.indexOf(err.type) >= 0
+        )
+      ) {
         return retryHandler(err)
       } else {
         throw err
       }
     })
   }, opts.retry === false ? { retries: 0 } : opts.retry).catch(err => {
-    if (err.status >= 500 || err.status === 408) {
+    if (err.status >= 400) {
       return err
     } else {
       throw err