invoke: correctly infer version for 0.1.0-vintage plugins

Older plugins return a known error when issued the VERSION command.
Capture this error and report it as a 0.1.0 version plugin.

invoke/exec_test.go

@@ -25,7 +25,7 @@ import (
 	. "github.com/onsi/gomega"
 )
 
-var _ = Describe("Executing a plugin", func() {
+var _ = Describe("Executing a plugin, unit tests", func() {
 	var (
 		pluginExec     *invoke.PluginExec
 		rawExec        *fakes.RawExec