Catch bad characters in rpkiManifest filenames earlier on

Catch bad characters in rpkiManifest filenames earlier on

This improves the hard-to-read error:

rpki-client: .rrdp/59B96A4C078FDCEDBB776D5BE8DF45EAC0149157547270EA7D4647A76611E145/rpki-rsync.us-east-2.amazonaws.com/volume/220c3ec2-ccf9-4b8a-bf61-fd4d1e151271/LAXNBPgDnLLjagP8++RFIoaMCGo.mft: RFC 6487 section 4.8.6: CRL: bad CRL distribution point extension
rpki-client: rpki-rsync.us-east-2.amazonaws.com/volume/220c3ec2-ccf9-4b8a-bf61-fd4d1e151271/LAXNBPgDnLLjagP8++RFIoaMCGo.mft: no valid mft available

to:

rpki-client: rpki.ripe.net/repository/DEFAULT/ZMvVW3ZpjFaCVe2TtDEqMlyFk3E.cer: SIA: rpkiManifest filename contains invalid characters

OK tb@