Added "Unknown toolchain/gem" error message

tasks/mruby_build.rake

@@ -29,10 +29,6 @@ module MRuby
       conf.instance_eval(&@initializer)
     end
 
-    def toolchain(name)
-      @@toolchains[name.to_s].setup(self)
-    end
-
     def self.load
       Dir.glob("#{File.dirname(__FILE__)}/toolchains/*.rake").each do |file|
         Kernel.load file
@@ -93,7 +89,9 @@ module MRuby
     end
 
     def toolchain(name)
-      Toolchain.toolchains[name.to_s].setup(self)
+      tc = Toolchain.toolchains[name.to_s]
+      fail "Unknown #{name} toolchain" unless tc
+      tc.setup(self)
     end
 
     def build_dir

tasks/mruby_build_gem.rake

@@ -2,7 +2,11 @@ module MRuby
   module LoadGems
     def gem(gemdir, &block)
       gemdir = load_external_gem(gemdir) if gemdir.is_a?(Hash)
-      load File.join(gemdir, "mrbgem.rake")
+      gemrake = File.join(gemdir, "mrbgem.rake")
+
+      fail "Can't find #{gemrake}" unless File.exists?(gemrake)
+      load gemrake
+
       Gem.current.dir = gemdir
       Gem.current.build = MRuby::Build.current
       Gem.current.build_config_initializer = block