more helpful error messages for legacy correlator issues.

[d3v-null]

instead of NAXIS2 in first gpubox image HDU 32 does not match expected value 128 (metafits fine chans per coarse [128])

it should point to the correct wiki article on the issue. https://mwatelescope.atlassian.net/wiki/spaces/MP/pages/24969295/Legacy+Correlator+Issues#LegacyCorrelatorIssues-NAXIS2Mismatch

steps to reproduce

birli *_gpubox??_*.fits -m 1221838184.metafits -u 1221838184_birli.uvfits