core: raise user-friendly errors if capabilities exception in Guest

lib/vagrant/guest.rb

@@ -32,6 +32,10 @@ module Vagrant
     def detect!
       guest_name = @machine.config.vm.guest
       initialize_capabilities!(guest_name, @guests, @capabilities, @machine)
+    rescue Errors::CapabilityHostExplicitNotDetected => e
+      raise Errors::GuestExplicitNotDetected, value: e.extra_data[:value]
+    rescue Errors::CapabilityHostNotDetected
+      raise Errors::GuestNotDetected
     end
 
     # Returns the specified or detected guest type name.

test/unit/vagrant/guest_test.rb

@@ -34,6 +34,18 @@ describe Vagrant::Guest do
 
       subject.detect!
     end
+
+    it "raises a user-friendly error if specified guest doesn't exist" do
+      machine.config.vm.stub(guest: :foo)
+
+      expect { subject.detect! }.
+        to raise_error(Vagrant::Errors::GuestExplicitNotDetected)
+    end
+
+    it "raises a user-friendly error if auto-detected guest not found" do
+      expect { subject.detect! }.
+        to raise_error(Vagrant::Errors::GuestNotDetected)
+    end
   end
 
   describe "#name" do