[posix-app] enhance RCP failure info (#3950)

By default, posix-app disables platform region logs. Even when
platform logs are enabled, logs	are sent to syslog without printing to
stderr.

This commit prints error information to	stderr on exit, providing the
user more direct information about failures. This commit also
eliminates the "wait no child process" error message.

src/posix/platform/hdlc_interface.cpp

@@ -171,8 +171,8 @@ void HdlcInterface::Deinit(void)
 {
     VerifyOrExit(mSockFd != -1);
 
-    VerifyOrExit(0 == close(mSockFd), perror("close NCP"));
-    VerifyOrExit(-1 != wait(NULL), perror("wait NCP"));
+    VerifyOrExit(0 == close(mSockFd), perror("close RCP"));
+    VerifyOrExit(-1 != wait(NULL) || errno == ECHILD, perror("wait RCP"));
 
     mSockFd = -1;
 

src/posix/platform/misc.c

@@ -112,6 +112,8 @@ void SuccessOrDie(otError aError)
     }
 
     otLogCritPlat("Error: %s", otThreadErrorToString(aError));
+    // For better user experience.
+    fprintf(stderr, "Error: %s\r\n", otThreadErrorToString(aError));
     exit(exitCode);
 }