perf buildid-cache: Warn --purge-all failures

Warn perf buildid-cache --purge-all failures in non verbose mode.

Ex.:

  $ sudo chown root:root /home/ravi/.debug -R
  $ sudo chmod 700 /home/ravi/.debug/ -R
  $ ./perf buildid-cache -P
    Couldn't remove some caches. Error: Permission denied.
Suggested-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20180510043651.12189-1-ravi.bangoria@linux.ibm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-buildid-cache.c

@@ -488,8 +488,12 @@ int cmd_buildid_cache(int argc, const char **argv)
 		}
 	}
 
-	if (purge_all)
-		ret = build_id_cache__purge_all();
+	if (purge_all) {
+		if (build_id_cache__purge_all()) {
+			pr_warning("Couldn't remove some caches. Error: %s.\n",
+				str_error_r(errno, sbuf, sizeof(sbuf)));
+		}
+	}
 
 	if (missing_filename)
 		ret = build_id_cache__fprintf_missing(session, stdout);