build: Fix clean checkout with pretty build log mode

`BuildExecutor#system` should not return anything, but it raises an
exception when the command fails. So, we should not use handle command
failure in caller side.

lib/ruby_wasm/build.rb

@@ -74,6 +74,7 @@ def system(*args, chdir: nil, env: nil)
             printer.done
           end
         end
+      return
     rescue => e
       $stdout.flush
       $stderr.puts "Try running with `rake --verbose` for more complete output."

lib/ruby_wasm/build/product/ruby_source.rb

@@ -51,13 +51,13 @@ def fetch(executor)
           "origin",
           "#{@params[:rev]}:origin/#{@params[:rev]}",
           chdir: src_dir
-        ) or raise "failed to clone #{repo_url}"
+        )
         executor.system(
           "git",
           "checkout",
           "origin/#{@params[:rev]}",
           chdir: src_dir
-        ) or raise "failed to checkout #{@params[:rev]}"
+        )
       when "local"
         executor.mkdir_p File.dirname(src_dir)
         executor.cp_r @params[:src], src_dir
@@ -80,10 +80,8 @@ def build(executor)
                         "gnu",
                         "config.guess",
                         "config.sub",
-                        chdir: src_dir or
-          raise "failed to download config.guess and config.sub"
-        executor.system "./autogen.sh", chdir: src_dir or
-          raise "failed to run autogen.sh"
+                        chdir: src_dir
+        executor.system "./autogen.sh", chdir: src_dir
       end
     end
   end