trivial_example: Fix error message

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>

doc/trivial_example.c

@@ -88,7 +88,7 @@ int main(int argc, char **argv)
    fin = fopen(inFile, "r");
    if (fin==NULL)
    {
-      fprintf(stderr, "failed to open file: %s\n", strerror(errno));
+      fprintf(stderr, "failed to open input file: %s\n", strerror(errno));
       return EXIT_FAILURE;
    }
 
@@ -104,7 +104,7 @@ int main(int argc, char **argv)
    fout = fopen(outFile, "w");
    if (fout==NULL)
    {
-      fprintf(stderr, "failed to open file: %s\n", strerror(errno));
+      fprintf(stderr, "failed to open output file: %s\n", strerror(errno));
       return EXIT_FAILURE;
    }
 
@@ -138,7 +138,7 @@ int main(int argc, char **argv)
       frame_size = opus_decode(decoder, cbits, nbBytes, out, MAX_FRAME_SIZE, 0);
       if (frame_size<0)
       {
-         fprintf(stderr, "decoder failed: %s\n", opus_strerror(err));
+         fprintf(stderr, "decoder failed: %s\n", opus_strerror(frame_size));
          return EXIT_FAILURE;
       }