Push dedicated BumpBlocks to the tail of the blocks list

Push dedicated BumpBlocks to the tail of the blocks list

BumpContext relies on using the head block from its 'blocks' field to
use as the current block to allocate new chunks to.  When we receive an
allocation request larger than allocChunkLimit, we place these chunks on
a new dedicated block and, until now, we pushed the block onto the
*head* of the 'blocks' list.

This behavior caused the previous bump block to no longer be available
for new normal-sized (non-large) allocations and would result in blocks
only being partially filled if a large allocation request arrived before
the block became full.

Here adjust the code to push these dedicated blocks onto the *tail* of
the blocks list so that the head block remains intact and available to
be used by normal allocation request sizes until it becomes full.

In passing, make the elog(ERROR) calls for the unsupported callbacks
consistent.  Likewise for the header comments for those functions.

Discussion: https://postgr.es/m/CAApHDvp9___r-ayJj0nZ6GD3MeCGwGZ0_6ZptWpwj+zqHtmwCw@mail.gmail.com
Discussion: https://postgr.es/m/CAApHDvqerXpzUnuDQfUEi3DZA+9=Ud9WSt3ruxN5b6PcOosx2g@mail.gmail.com