raise when using generate_from_image without RMagick

lib/paleta.rb

@@ -3,6 +3,8 @@
 require 'paleta/palette'
 
 module Paleta
+  class MissingDependencyError < StandardError; end
+
   @rmagick_available = begin
     require "rmagick"
   rescue LoadError

lib/paleta/palette.rb

@@ -211,7 +211,7 @@ def fit
 
     def self.generate_from_image(path, size = 5)
       unless Paleta.rmagick_available?
-        return puts "You must install RMagick to use Palette.generate(:from => :image, ...)"
+        raise Paleta::MissingDependencyError, "You must install RMagick to use Palette.generate(:from => :image, ...)"
       end
 
       begin

spec/models/palette_spec.rb

@@ -231,6 +231,17 @@
     palette.size.should == size
   end
 
+  it "should raise when generating a Palette from an image without RMagick" do
+    # stub Paleta.rmagick_available? to return false
+    allow(Paleta).to receive(:rmagick_available?).and_return(false)
+
+    path = File.join(File.dirname(__FILE__), '..', 'images/test.jpg')
+    size = 5
+    expect do
+      Paleta::Palette.generate(:from => :image, :image => path, :size => size)
+    end.to raise_error(Paleta::MissingDependencyError)
+  end
+
   it "should raise an error when generating a Palette from an invalid image" do
     expect{ Paleta::Palette.generate(:from => :image, :image => "/no/image.here") }.to raise_error(RuntimeError)
   end