JSON parser: Improve by using exceptions to get exact error

Also strip redundant "_Parser_" prefix from private methods.
JSON parser: Fixed increasing position after constants.

The first letter is already increased by the main parse loop.
JSON parser: Added TODOs.
HaikuDepot/WebAppInterface: Support bulk transfers.

Refactor code to send different JSON requests.
HaikuDepot: Use web app bulk transfers.

Collect up to 50 packages and get information about them in a bulk transfer.
If that fails, do it in two smaller transfers, until only one package is
left, fall back to use the getPkg method than.

Remember packages for which there is an icon on the server. After the
first round of reading icons from the cache, only try to fetch icons from
the server for which we know one should be there.

Added quite a bit of command line output to see what is going on.
HaikuDepot: Add User-Agent header to web app JSON requests.