mosh.pl: Report errors on server command failure.

I think this will help with the papercut of "Did not find mosh server
startup message".  This message confuses people; often the real
problem is that the command to start the remote mosh-server failed
somehow.

Fixes mobile-shell#1042, partially resolves mobile-shell#1005.

Also relevant for mobile-shell#1042 and countless questions on IRC.

scripts/mosh.pl

@@ -350,8 +350,7 @@ sub predict_check {
   $userhost = "$user$ip";
 }
 
-# Construct exec arguments.
-
+# Construct server exec arguments.
 my @sshopts = ( '-n' );
 if ($ssh_pty) {
   push @sshopts, '-tt';
@@ -450,8 +449,13 @@ sub predict_check {
     print "$_\n";
   }
 }
-waitpid $pid, 0;
-close $pipe;
+if ( not close $pipe ) {
+  if ( $! == 0 ) {
+    die_on_exitstatus($?, "server command", shell_quote(@exec_argv));
+  } else {
+    die("$0: error closing server pipe: $!\n")
+  }
+}
 
 if ( not defined $ip ) {
   if ( defined $sship ) {
@@ -546,3 +550,20 @@ sub resolvename {
   }
   return @res;
 }
+
+sub die_on_exitstatus {
+  my ($exitstatus, $what_command, $exec_string) = @_;
+
+  if (POSIX::WIFSIGNALED($exitstatus)) {
+    my $termsig = POSIX::WTERMSIG($exitstatus);
+    die("$0: $what_command exited on signal $termsig: $exec_string\n" );
+  }
+  if (!POSIX::WIFEXITED($exitstatus)) {
+    die("$0: $what_command unexpectedly terminated with exit status $exitstatus: $exec_string\n");
+  }
+  my $exitcode = POSIX::WEXITSTATUS($exitstatus);
+  if ($exitcode != 0) {
+    die("$0: $what_command failed with exitstatus $exitcode: $exec_string\n");
+  }
+  return;
+}