Handle structured error codes.

src/hci/strerror.c

 	__table_end ( struct errortab, errortab );
 
 /**
- * Retrieve string representation of error number.
+ * Find error description
+ *
+ * @v errno		Error number
+ * @v mask		Mask of bits that we care about
+ * @ret errortab	Error description, or NULL
+ */
+static struct errortab * find_error ( int errno, int mask ) {
+	struct errortab *errortab;
+
+	for ( errortab = errortab_start ; errortab < errortab_end ;
+	      errortab++ ) {
+		if ( ( ( errortab->errno ^ errno ) & mask ) == 0 )
+			return errortab;
+	}
+
+	return NULL;
+}
+
+/**
+ * Find closest error description
  *
  * @v errno		Error number
+ * @ret errortab	Error description, or NULL
+ *
+ * 
+ */
+static struct errortab * find_closest_error ( int errno ) {
+	struct errortab *errortab;
+
+	/* First, look for an exact match */
+	if ( ( errortab = find_error ( errno, 0x7fffffff ) ) != NULL )
+		return errortab;
+
+	/* Second, try masking off the gPXE-specific bit and seeing if
+	 * we have an entry for the generic POSIX error message.
+	 */
+	if ( ( errortab = find_error ( errno, 0x0000ffff ) ) != NULL )
+		return errortab;
+
+	/* Lastly, try masking off the POSIX bits and seeing if we
+	 * have a match just based on the PXENV component.  This
+	 * allows us to report errors from underlying PXE stacks.
+	 */
+	if ( ( errortab = find_error ( ( errno & 0x000000ff ),
+				       0xffff00ff ) ) != NULL )
+		return errortab;
+
+	return NULL;
+}
+
+/**
+ * Retrieve string representation of error number.
+ *
+ * @v errno/rc		Error number or return status code
  * @ret strerror	Pointer to error text
  *
  * If the error is not found in the linked-in error tables, generates
  *
  */
 const char * strerror ( int errno ) {
-	static char *generic_message = "Error 0x0000";
+	static char errbuf[64];
 	struct errortab *errortab;
 
 	/* Allow for strerror(rc) as well as strerror(errno) */
 	if ( errno < 0 )
 		errno = -errno;
 
-	for ( errortab = errortab_start ; errortab < errortab_end ;
-	      errortab++ ) {
-		if ( errortab->errno == errno )
-			return errortab->text;
+	/* Find the error description, if one exists */
+	errortab = find_closest_error ( errno );
+
+	/* Construct the error message */
+	if ( errortab ) {
+		snprintf ( errbuf, sizeof ( errbuf ), "%s (%#08x)",
+			   errortab->text, errno );
+	} else {
+		snprintf ( errbuf, sizeof ( errbuf ), "Error %#08x", errno );
 	}
 
-	sprintf ( generic_message + 8, "%hx", errno );
-	return generic_message;
+	return errbuf;
 }
 
 /** The most common errors */