[segment] Add "Requested memory not available" error message

prep_segment() can sometimes fail because an image requests memory
that is already in use by gPXE.  This will happen if
e.g. undionly.kpxe is used to boot memtest86; the memtest86 image is
an old-format kernel that needs to be loaded at 9000:0000, but this
area of memory may well already be in use by the underlying PXE stack.

Add a human-friendly error message, so that the cause is more
immediately visible.

src/hci/strerror.c

 	{ ENOSPC, "No space left on device" },
 	{ ENOTSUP, "Not supported" },
 	{ EPERM, "Operation not permitted" },
+	{ ERANGE, "Out of range" },
 	{ ETIMEDOUT, "Connection timed out" },
 };

src/image/segment.c

 #include <errno.h>
 #include <gpxe/uaccess.h>
 #include <gpxe/memmap.h>
+#include <gpxe/errortab.h>
 #include <gpxe/segment.h>
 
 /**
 	      start, mid, end );
 	return -ERANGE;
 }
+
+/**
+ * Segment-specific error messages
+ *
+ * This error happens sufficiently often to merit a user-friendly
+ * description.
+ */
+struct errortab segment_errors[] __errortab = {
+	{ ERANGE, "Requested memory not available" },
+};