SystemError: Fix inconsistent signature.

spec/std/system_error_spec.cr

@@ -0,0 +1,11 @@
+require "spec"
+
+describe SystemError do
+  describe ".from_os_error" do
+    it "Can build an error from an errno" do
+      errno = Errno.new(2)
+      error = ::IO::Error.from_os_error(message: nil, os_error: errno)
+      error.message.should eq("No such file or directory")
+    end
+  end
+end

src/system_error.cr

@@ -85,7 +85,7 @@ module SystemError
     #
     # By default it returns the original message unchanged. But that could be
     # customized based on the keyword arguments passed to `from_errno` or `from_winerror`.
-    protected def build_message(message, **opts) : String
+    protected def build_message(message : String?, **opts) : String?
       message
     end