perf report: Print more expressive message in case of file open error

Before:

 $ perf report
 failed to open file: No such file or directory

After:

 $ perf report
  failed to open file: perf.data  (try 'perf record' first)

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

tools/perf/builtin-report.c

@@ -1120,7 +1120,10 @@ static int __cmd_report(void)
 
 	input = open(input_name, O_RDONLY);
 	if (input < 0) {
-		perror("failed to open file");
+		fprintf(stderr, " failed to open file: %s", input_name);
+		if (!strcmp(input_name, "perf.data"))
+			fprintf(stderr, "  (try 'perf record' first)");
+		fprintf(stderr, "\n");
 		exit(-1);
 	}