[http] Include error messages for 4xx and 5xx response codes

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp/httpcore.c

 #include <ipxe/params.h>
 #include <ipxe/profile.h>
 #include <ipxe/vsprintf.h>
+#include <ipxe/errortab.h>
 #include <ipxe/http.h>
 
 /* Disambiguate the various error causes */
 /** Data transfer profiler */
 static struct profiler http_xfer_profiler __profiler = { .name = "http.xfer" };
 
+/** Human-readable error messages */
+struct errortab http_errors[] __errortab = {
+	__einfo_errortab ( EINFO_EIO_4XX ),
+	__einfo_errortab ( EINFO_EIO_5XX ),
+};
+
 static struct http_state http_request;
 static struct http_state http_headers;
 static struct http_state http_trailers;