Improve error message for MaxAllocSize overrun in accumArrayResult.

Before, if you went past about 64M array elements in array_agg() and
allied functions, you got a generic "invalid memory alloc request
size" error.  This patch replaces that with "array size exceeds the
maximum allowed", which seems more user-friendly since it points you
to needing to reduce the size of your array result.  (This is the
same error text you'd get from construct_md_array in the event of
overrunning the maximum physical size for the finished array.)

Per question from Shaozhong Shi.  Since this hasn't come up often,
I don't feel a need to back-patch.

Discussion: https://postgr.es/m/CA+i5JwYtVS9z2E71PcNKAVPbOn4R2wuj-LqbJsYr_XOz73q7dQ@mail.gmail.com

src/backend/utils/adt/arrayfuncs.c

@@ -5317,6 +5317,12 @@ accumArrayResult(ArrayBuildState *astate,
 	if (astate->nelems >= astate->alen)
 	{
 		astate->alen *= 2;
+		/* give an array-related error if we go past MaxAllocSize */
+		if (!AllocSizeIsValid(astate->alen * sizeof(Datum)))
+			ereport(ERROR,
+					(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
+					 errmsg("array size exceeds the maximum allowed (%d)",
+							(int) MaxAllocSize)));
 		astate->dvalues = (Datum *)
 			repalloc(astate->dvalues, astate->alen * sizeof(Datum));
 		astate->dnulls = (bool *)