Improve error reporting/handling when reading LIGO_LW Arrays
This PR fixes an issue seen by @rmacas when reading <Array>
elements from LIGO_LW files. Fundamentally the problem was just that the wrong error message was printed when multiple <Array>
elements were found and gwpy didn't know how to pick the right one (fixed in 288c3a20), but users probably require a wee bit more help to understand how to modify their code to avoid the error in the first place.