print an error message for invalid path

print an error message for invalid path

If verification of path failed, it is always better to print an
error message saying this than relying on the caller function to
print a meaningful error message (especially when the callee already
prints error message for another situation).

Because the callers of add_index_entry_with_check() did not print
any error message, it resulted that the user would not notice the
problem when checkout of an invalid path failed.

Signed-off-by: Dmitry Potapov <dpotapov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>