Add logging for errors about loading dependencies on startup

[daipom]

Which issue(s) this PR fixes:
None.

What this PR does / why we need it:
Adds error handling similar to worker process.

fluentd/lib/fluent/supervisor.rb

Lines 1149 to 1164 in 8df1086

	rescue ScriptError => e # LoadError, NotImplementedError, SyntaxError
	logging_with_console_output do |log|
	if e.respond_to?(:path)
	log.error e.message, path: e.path, error: e
	else
	log.error e.message, error: e
	end
	log.error_backtrace
	end
	unrecoverable_error = true
	rescue => e
	logging_with_console_output do |log|
	log.error "unexpected error", error: e
	log.error_backtrace
	end
	end

Fluent::Engine.run_configure calls Fluent::Registry#search.
It can run Kernel.require.
It can raise LoadError, ConflictError and others.

Without this error handling, Fluentd cannot log those errors.
This makes a situation where Fluentd fails to start, but there is no error log in the log file.
This solves that problem.

This appears to be an oversight of the following fix:

• Fix dry run mode #2651

Docs Changes:
Not needed.

Release Note:
The same as the title.

[Watson1978]

👍🏻