throw better error messages for defensive implicit AbstractRecord convert failures

[jrevels]

we should suggest that the caller explicitly constructs relevant Rows instead of relying on implicit conversion

ref #27 (comment)

In other words, I think in the example in the OP the caller should explicitly push!(signals::Vector{<:Signal}, Signal(nt)); depending on the situation the error that gets raised otherwise is actually "a feature" and not just an inconvenience depending on the caller's actual intentions, so we should continue to force them to express those intentions explicitly

In that case I'd advocate for defining convert and having it raise an error message explaining that intent