lh5concat silently drops data it can't merge

[ManuelHu]

background: after legend-exp/remage#152 remage produces LH5 with tables nested into a top-level struct.

After testing with those files, @gipert found that lh5concat does nothing at all when working on those files. (*)

1. lh5concat does not merge all LGDO types, e.g. structs
   of course this can only be fixed for "group-like" structs (i.e. normal HDF5 group with a set datatype attr, only containing Array-like datasets or other groups/group-like structs)
2. but it also does not warn about data being "discarded" or not included in this process.
   this is escpecially bad as the user does not see anything from this error condition, and might wonder why nothing happened or why data is missing from merged files.