perf session: fix error message on failure to open perf.data

If we cannot open our data file, print strerror(errno) for a more
comprehensible error message; and only suggest 'perf record' on ENOENT.

In particular, this fixes the nonsensical advice when:

    % sudo perf record sleep 1
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.009 MB perf.data (~381 samples) ]
    % perf trace
    failed to open file: perf.data  (try 'perf record' first)
    %

Cc: Ingo Molnar <mingo@elte.hu>
LPU-Reference: <20100612033615.GA24731@hexapodia.org>
Signed-off-by: Andy Isaacson <adi@hexapodia.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/session.c

@@ -27,8 +27,10 @@ static int perf_session__open(struct perf_session *self, bool force)
 
 	self->fd = open(self->filename, O_RDONLY);
 	if (self->fd < 0) {
-		pr_err("failed to open file: %s", self->filename);
-		if (!strcmp(self->filename, "perf.data"))
+		int err = errno;
+
+		pr_err("failed to open %s: %s", self->filename, strerror(err));
+		if (err == ENOENT && !strcmp(self->filename, "perf.data"))
 			pr_err("  (try 'perf record' first)");
 		pr_err("\n");
 		return -errno;