[ifmgmt] Include human-readable error message for configuration failure

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

src/usr/ifmgmt.c

 #include <ipxe/job.h>
 #include <ipxe/monojob.h>
 #include <ipxe/timer.h>
+#include <ipxe/errortab.h>
 #include <usr/ifmgmt.h>
 
 /** @file
 	__einfo_uniqify ( EINFO_EADDRNOTAVAIL, 0x01,			\
 			  "No configuration methods succeeded" )
 
+/** Human-readable error message */
+struct errortab ifmgmt_errors[] __errortab = {
+	__einfo_errortab ( EINFO_EADDRNOTAVAIL_CONFIG ),
+};
+
 /**
  * Open network device
  *