auth-type: gracefully fallback from web

Originally `auth-type=web` would gracefully fallback to
`auth-type=legacy` if a registry had not yet implemented
web. This brings back that behavior.

lib/utils/auth.js

@@ -4,6 +4,26 @@ const openUrlPrompt = require('../utils/open-url-prompt.js')
 const read = require('../utils/read-user-info.js')
 const otplease = require('../utils/otplease.js')
 
+async function loginCouch (npm, creds, opts) {
+  const username = await read.username('Username:', creds.username)
+  const password = await read.password('Password:', creds.password)
+  return await otplease(npm, opts, (reqOpts) =>
+    profile.loginCouch(username, password, reqOpts)
+  )
+}
+
+async function addUserCouch (npm, creds, opts) {
+  const username = await read.username('Username:', creds.username)
+  const password = await read.password('Password:', creds.password)
+  const email = await read.email('Email: (this IS public) ', creds.email)
+  // npm registry quirk: If you "add" an existing user with their current
+  // password, it's effectively a login, and if that account has otp you'll
+  // be prompted for it.
+  return await otplease(npm, opts, (reqOpts) =>
+    profile.adduserCouch(username, email, password, opts)
+  )
+}
+
 const adduser = async (npm, { creds, ...opts }) => {
   const authType = npm.config.get('auth-type')
   let res
@@ -16,17 +36,16 @@ const adduser = async (npm, { creds, ...opts }) => {
         'Press ENTER to open in the browser...',
         emitter
       )
-    }, opts)
+    }, opts).catch(er => {
+      if (er.message === 'Web login not supported' && er.code === 'ENYI') {
+        log.verbose('web add user not supported, trying couch')
+        return addUserCouch(npm, creds, opts)
+      } else {
+        throw er
+      }
+    })
   } else {
-    const username = await read.username('Username:', creds.username)
-    const password = await read.password('Password:', creds.password)
-    const email = await read.email('Email: (this IS public) ', creds.email)
-    // npm registry quirk: If you "add" an existing user with their current
-    // password, it's effectively a login, and if that account has otp you'll
-    // be prompted for it.
-    res = await otplease(npm, opts, (reqOpts) =>
-      profile.adduserCouch(username, email, password, opts)
-    )
+    res = addUserCouch(npm, creds, opts)
   }
 
   // We don't know the username if it was a web login, all we can reliably log is scope and registry
@@ -52,13 +71,16 @@ const login = async (npm, { creds, ...opts }) => {
         'Press ENTER to open in the browser...',
         emitter
       )
-    }, opts)
+    }, opts).catch(er => {
+      if (er.message === 'Web login not supported' && er.code === 'ENYI') {
+        log.verbose('web login not supported, trying couch')
+        return loginCouch(npm, creds, opts)
+      } else {
+        throw er
+      }
+    })
   } else {
-    const username = await read.username('Username:', creds.username)
-    const password = await read.password('Password:', creds.password)
-    res = await otplease(npm, opts, (reqOpts) =>
-      profile.loginCouch(username, password, reqOpts)
-    )
+    res = loginCouch(npm, creds, opts)
   }
 
   // We don't know the username if it was a web login, all we can reliably log is scope and registry