communicators/ssh: clean error on max session [GH-4044]

CHANGELOG.md

@@ -28,6 +28,7 @@ BUG FIXES:
   - commands/package: base package won't crash with exception [GH-4017]
   - commands/rsync-auto: Destroyed machines won't raise exceptions. [GH-4031]
   - communicators/ssh: Nicer error if remote unexpectedly disconects. [GH-4038]
+  - communicators/ssh: Clean error when max sessions is hit. [GH-4044]
   - communicators/winrm: Support `mkdir` [GH-4271]
   - guests/centos: Fix issues when NFS client is installed by restarting
       NFS [GH-4088]

lib/vagrant/errors.rb

@@ -576,6 +576,10 @@ module Vagrant
       error_key(:ssh_authentication_failed)
     end
 
+    class SSHChannelOpenFail < VagrantError
+      error_key(:ssh_channel_open_fail)
+    end
+
     class SSHConnectEACCES < VagrantError
       error_key(:ssh_connect_eacces)
     end

plugins/communicators/ssh/communicator.rb

@@ -520,6 +520,8 @@ module VagrantPlugins
             @logger.info(
               "SSH connection unexpected closed. Assuming reboot or something.")
             exit_status = 0
+          rescue Net::SSH::ChannelOpenFailed
+            raise Vagrant::Errors::SSHChannelOpenFail
           rescue Net::SSH::Disconnect
             raise Vagrant::Errors::SSHDisconnected
           end

templates/locales/en.yml

@@ -973,6 +973,11 @@ en:
         assumes that this means the command failed. The output for this command
         should be in the log above. Please read the output to determine what
         went wrong.
+      ssh_channel_open_fail: |-
+        Failed to open an SSH channel on the remote end! This typically
+        means that the maximum number of active sessions was hit on the
+        SSH server. Please configure your remote SSH server to resolve
+        this issue.
       ssh_connect_eacces: |-
         SSH is getting permission denied errors when attempting to connect
         to the IP for SSH. This is usually caused by network rules and not being