Raise exception if provider doesn't have snapshot capability

Prior to this commit, if a user attempted to use the `vagrant snapshot
save` or `vagrant snapshot list` commands on a vm whose provider did not
support snapshots, it would simply print a warning. This commit changes
that behavior by instead raising an error.

lib/vagrant/errors.rb

@@ -656,6 +656,10 @@ module Vagrant
       error_key(:snapshot_force)
     end
 
+    class SnapshotNotSupported < VagrantError
+      error_key(:snapshot_not_supported)
+    end
+
     class SSHAuthenticationFailed < VagrantError
       error_key(:ssh_authentication_failed)
     end

plugins/commands/snapshot/command/list.rb

@@ -22,8 +22,7 @@ module VagrantPlugins
             end
 
             if !vm.provider.capability?(:snapshot_list)
-              vm.ui.info(I18n.t("vagrant.commands.snapshot.not_supported"))
+              raise Vagrant::Errors::SnapshotNotSupported
-              next
             end
 
             snapshots = vm.provider.capability(:snapshot_list)

plugins/commands/snapshot/command/save.rb

@@ -34,8 +34,7 @@ module VagrantPlugins
           name = argv.pop
           with_target_vms(argv) do |vm|
             if !vm.provider.capability?(:snapshot_list)
-              vm.ui.info(I18n.t("vagrant.commands.snapshot.not_supported"))
+              raise Vagrant::Errors::SnapshotNotSupported
-              next
             end
 
             snapshot_list = vm.provider.capability(:snapshot_list)

templates/locales/en.yml

@@ -1158,6 +1158,12 @@ en:
         guest system. Please report a bug with your Vagrantfile and debug log.
       snapshot_force: |-
         You must include the `--force` option to replace an existing snapshot.
+      snapshot_not_supported: |-
+        This provider doesn't support snapshots.
+
+        This may be intentional or this may be a bug. If this provider
+        should support snapshots, then please report this as a bug to the
+        maintainer of the provider.
       ssh_authentication_failed: |-
         SSH authentication failed! This is typically caused by the public/private
         keypair for the SSH user not being properly set on the guest VM. Please

test/unit/plugins/commands/snapshot/command/save_test.rb

@@ -38,6 +38,21 @@ describe VagrantPlugins::CommandSnapshot::Command::Save do
       end
     end
 
+    context "with an unsupported provider" do
+      let(:argv)     { ["test"] }
+
+      before do
+        allow(machine.provider).to receive(:capability?).with(:snapshot_list).
+          and_return(false)
+      end
+
+      it "raises an exception" do
+        machine.id = "foo"
+        expect { subject.execute }.
+          to raise_error(Vagrant::Errors::SnapshotNotSupported)
+      end
+    end
+
     context "with a snapshot name given" do
       let(:argv)     { ["test"] }
       it "calls snapshot_save with a snapshot name" do