Matroska reader, kax_file_c: improve exception messages

See #1089.
mkvinfo: test returned element to be an instance of EbmlHead

See #1089.
build system: require libEBML 1.3.1

Together with 563be4d and 8415af3 the changes in libEBML 1.3.1 address
the problems in #1089. Therefore: fixes #1089.
libEBML, libMatroska: update to current HEADs in origin repositories