[FfmpegAdapter] #write_errors should create exit code file to indicate failed status

[cjcolvar]

active_encode/lib/active_encode/engine_adapters/ffmpeg_adapter.rb

Lines 197 to 199 in f35a797

	def write_errors(encode)
	File.write(working_path("error.log", encode.id), encode.errors.join("\n"))
	end

#write_errors writes out an error.log file but not an exit_status.code which gets used in determining the status in the #find method

active_encode/lib/active_encode/engine_adapters/ffmpeg_adapter.rb

Lines 135 to 137 in f35a797

	exit_code = read_exit_code(id)
	if exit_code.present? && exit_code != 0 && exit_code != 143
	encode.state = :failed

This leads to an issue along with #129 when a file fails in #create during inspection.

Alternatively the #find if statements could be changed to check for error.log. Although this might not work since the ffmpeg command outputs to error.log for stderr which might happen even when the command is still running or succeeds.