also add a friendly error for mismatch

Author: mcabbott

src/data/dataloader.jl

@@ -109,8 +109,10 @@ _nobs(data::AbstractArray) = size(data)[end]
 function _nobs(data::Union{Tuple, NamedTuple})
     length(data) > 0 || throw(ArgumentError("Need at least one data input"))
     n = _nobs(data[1])
-    if !all(x -> _nobs(x) == n, Base.tail(data))
-        throw(DimensionMismatch("All data should contain same number of observations"))
+    for i in keys(data)
+        ni = _nobs(data[i])
+        n == ni || throw(DimensionMismatch("All data inputs should have the same number of observations, i.e. size in the last dimension. " * 
+            "But data[$(repr(first(keys(data))))] ($(summary(data[1]))) has $n, while data[$(repr(i))] ($(summary(data[i]))) has $ni."))
     end
     return n
 end