(#32) Handle vmpooler responses when token is invalid

This commit adds an errors class to handle when pooler tokens are
invalid. The retrieve method in Pooler will raise an AuthError if it
gets an HTTP 401 from the vmpooler.

lib/vmfloaty/errors.rb

@@ -0,0 +1,5 @@
+class AuthError < StandardError
+  def initialize(msg="Could not authenticate to pooler")
+    super
+  end
+end

lib/vmfloaty/pooler.rb

@@ -1,6 +1,7 @@
 require 'faraday'
 require 'vmfloaty/http'
 require 'json'
+require 'vmfloaty/errors'
 
 class Pooler
   def self.list(verbose, url, os_filter=nil)
@@ -40,10 +41,13 @@ class Pooler
     response = conn.post "vm/#{os_string}"
 
     res_body = JSON.parse(response.body)
+
     if res_body["ok"]
       res_body
+    elsif response.status == 401
+      raise AuthError, "HTTP #{response.status}: The token provided could not authenticate to the pooler.\n#{res_body}"
     else
-      raise "Failed to obtain VMs from the pooler at #{url}/vm/#{os_string}. #{res_body}"
+      raise "HTTP #{response.status}: Failed to obtain VMs from the pooler at #{url}/vm/#{os_string}. #{res_body}"
     end
   end