add aliases for Mysql compatibility

lib/mysql2/error.rb

@@ -7,5 +7,9 @@ module Mysql2
       @error_number = nil
       @sql_state    = nil
     end
+
+    # Mysql gem compatibility
+    alias_method :errno, :error_number
+    alias_method :error, :message
   end
 end

spec/mysql2/error_spec.rb

@@ -13,4 +13,13 @@ describe Mysql2::Error do
   it "should respond to #sql_state" do
     @error.should respond_to(:sql_state)
   end
+
+  # Mysql gem compatibility
+  it "should alias #error_number to #errno" do
+    @error.should respond_to(:errno)
+  end
+
+  it "should alias #message to #error" do
+    @error.should respond_to(:error)
+  end
 end