Improve some user facing AuthenticationError msgs

lib/signature.rb

@@ -99,10 +99,10 @@ def authenticate_by_token(token, timestamp_grace = 600)
     def authenticate(timestamp_grace = 600)
       raise ArgumentError, "Block required" unless block_given?
       key = @auth_hash['auth_key']
-      raise AuthenticationError, "Authentication key required" unless key
+      raise AuthenticationError, "Missing parameter: auth_key" unless key
       token = yield key
       unless token
-        raise AuthenticationError, "Invalid authentication key"
+        raise AuthenticationError, "Unknown auth_key"
       end
       authenticate_by_token!(token, timestamp_grace)
       return token

spec/signature_spec.rb

@@ -180,14 +180,14 @@
         request = Signature::Request.new('POST', '/some/path', @params)
         lambda {
           request.authenticate { |key| nil }
-        }.should raise_error('Authentication key required')
+        }.should raise_error('Missing parameter: auth_key')
       end
 
       it "should raise error if block returns nil (i.e. key doesn't exist)" do
         request = Signature::Request.new('POST', '/some/path', @params)
         lambda {
           request.authenticate { |key| nil }
-        }.should raise_error('Invalid authentication key')
+        }.should raise_error('Unknown auth_key')
       end
 
       it "should raise unless block given" do