Add logging for errors about loading dependencies on startup (#4858)

**Which issue(s) this PR fixes**: 
None.

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

https://github.com/fluent/fluentd/blob/8df10863769da32459aba2a1408134d0b8e7d9f4/lib/fluent/supervisor.rb#L1149-L1164

`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:

* #2651

**Docs Changes**:
Not needed.

**Release Note**: 
The same as the title.

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Author: daipom

lib/fluent/supervisor.rb

@@ -721,6 +721,18 @@ def run_supervisor(dry_run: false)
         $log.error 'config error', file: @config_path, error: e
         $log.debug_backtrace
         exit!(1)
+      rescue ScriptError => e # LoadError, NotImplementedError, SyntaxError
+        if e.respond_to?(:path)
+          $log.error e.message, path: e.path, error: e
+        else
+          $log.error e.message, error: e
+        end
+        $log.debug_backtrace
+        exit!(1)
+      rescue => e
+        $log.error "unexpected error", error: e
+        $log.debug_backtrace
+        exit!(1)
       end
 
       if dry_run

test/command/test_fluentd.rb

@@ -570,7 +570,7 @@ def assert_fluentd_fails_to_start(cmdline, *pattern_list, timeout: 20)
 
       assert_fluentd_fails_to_start(
         create_cmdline(conf_path, "-p", File.dirname(plugin_path)),
-        /in_buggy.rb:\d+:.+\(SyntaxError\)/
+        /\[error\]: .+in_buggy.rb:\d+: syntax error/
       )
     end
   end