Add error types, small comment typo

errors.go

@@ -0,0 +1,12 @@
+package main
+
+import "fmt"
+
+type UnexpectedContentTypeError struct {
+	Expected string
+	Actual   string
+}
+
+func (e *UnexpectedContentTypeError) Error() string {
+	return fmt.Sprintf("Expected content type '%s', received '%s'", e.Expected, e.Actual)
+}