[llvm-exegesis] Print errno on failures in subprocess

[aidengrossman]

Summary

Some error logging in llvm-exegesis under the subprocess executor just
prints a generic failure information rather than any details about the
error as we omit printing the string version of errno. This patch adds
in printing errno at all relevant points in the subprocess executor that
were previously missed.

[aidengrossman]

Missed a couple of these during the switch away from pidfd_getfd and that made debugging a little difficult when I had things setup incorrectly while pulling in Exegesis as a library in an external project.

I didn't add any lit tests here as I don't think there's a way to invoke these failure conditions without relying on buggy behavior.

[aidengrossman]

Bump on this when reviewers have a chance. Thanks!

llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp

Error sendFileDescriptorThroughSocket(int SocketFD, int FD) const {
 
     memcpy(CMSG_DATA(ControlMessage), &FD, sizeof(FD));
 
     Message.msg_controllen = CMSG_SPACE(sizeof(FD));
 
     ssize_t BytesWritten = sendmsg(SocketFD, &Message, 0);
 
     if (BytesWritten < 0)
-      return make_error<Failure>("Failed to write FD to socket");
+      return make_error<Failure>("Failed to write FD to socket: " +
+                                 Twine(strerror(errno)));
 
     return Error::success();
   }
 
   Expected<int> getFileDescriptorFromSocket(int SocketFD) const {
     struct msghdr Message = {};
 
     char ControlBuffer[256];
     Message.msg_control = ControlBuffer;
     Message.msg_controllen = sizeof(ControlBuffer);
 
     ssize_t BytesRead = recvmsg(SocketFD, &Message, 0);
 
     if (BytesRead < 0)
-      return make_error<Failure>("Failed to read FD from socket");
+      return make_error<Failure>("Failed to read FD from socket: " +
+                                 Twine(strerror(errno)));
 
     struct cmsghdr *ControlMessage = CMSG_FIRSTHDR(&Message);
 
     int FD;
 
     if (ControlMessage->cmsg_len != CMSG_LEN(sizeof(FD)))
       return make_error<Failure>("Failed to get correct number of bytes for "
                                  "file descriptor from socket.");
 
     memcpy(&FD, CMSG_DATA(ControlMessage), sizeof(FD));
 
     return FD;
   }
 
   Error createSubProcessAndRunBenchmark(
       StringRef CounterName, SmallVectorImpl<int64_t> &CounterValues) const {
     int PipeFiles[2];
     int PipeSuccessOrErr = socketpair(AF_UNIX, SOCK_DGRAM, 0, PipeFiles);
     if (PipeSuccessOrErr != 0) {
       return make_error<Failure>(
           "Failed to create a pipe for interprocess communication between "
-          "llvm-exegesis and the benchmarking subprocess");
+          "llvm-exegesis and the benchmarking subprocess: " +
+          Twine(strerror(errno)));
     }
 
     SubprocessMemory SPMemory;
     Error MemoryInitError = SPMemory.initializeSubprocessMemory(getpid());
     if (MemoryInitError)
       return MemoryInitError;
 
     Error AddMemDefError =