po: explain why `make update-po` will fail on a VPATH build.
Commit c9600691 worked around a still-existing bug (well at least the bug report is still open) of intltool with automake 1.15 when the build and source directories are different (VPATH build) by replacing `intltool-update` by `true`. But because of this `make update-po` fails on such a build and the make output is totally unclear at why. Add an error message when this happens.
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