objtool: Improve error message for bad file argument

If a nonexistent file is supplied to objtool, it complains with a
non-helpful error:

  open: No such file or directory

Improve it to:

  objtool: can't open file /.tmp_
  open: No such file or directory

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 2446015..cc56e7a 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -358,6 +358,7 @@
 
 	elf->fd = open(name, flags);
 	if (elf->fd == -1) {
+		fprintf(stderr, "objtool: can't open file %s\n", name);
 		perror("open");
 		goto err;
 	}