Improve wording describing error returns.

man2/futex.2

@@ -165,9 +165,8 @@ but first checks whether the location
 .I uaddr
 still contains the value
 .IR val3 .
-If not, an error
-.B EAGAIN
-is returned.
+If not, the operation fails with the error
+.BR EAGAIN .
 The argument
 .I timeout
 is ignored.
@@ -181,15 +180,15 @@ Returns 0 if the process was woken by a
 .B FUTEX_WAKE
 call.
 In case of timeout,
-.B ETIMEDOUT
-is returned.
+the operation fails with the error
+.BR ETIMEDOUT .
 If the futex was not equal to the expected value,
 the operation fails with the error
 .BR EWOULDBLOCK .
 Signals (or other spurious wakeups)
 cause
 .B FUTEX_WAIT
-to return
+to fail with the error
 .BR EINTR .
 .TP
 .B FUTEX_WAKE