[5.0] Report read-only API not enabled to caller

[heifner]

Report read-only transactions execution not enabled on API node. Set read-only-threads > 0 to caller when /v1/chain/send_read_only_transaction is not supported because of no configured read-only-threads. Note this is handled correctly in 4.0.x.

Includes some misc fixes found while debugging this issue.

• Log http errors before calling the callback cb, as cb may throw; e.g. cb not set.
• Add missing } in a producer_plugin error log, so the error is reported correctly.
• Add the response to the trx_provider error log when the http call fails.

[ericpassmore]

Note:start
group: CLEANCODE
category: INTERNALS
summary: Now report read-only API not enabled to caller, when read-only-threads is not configured.
Note:end