of: overlay: make pr_err() string unique

The same error string occurs in drivers/of/resolver.c.  Change
the error here to more precisely describe this case, and avoid
the possible confusion of looking in the wrong source location
to understand the cause of an error.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>