IAM role check doesn't catch all potential exceptions

[golenkovm]

When IAM role is not set it tries to connect without key and secreat and therefore throws \Aws\Exception\CredentialsException exception: https://github.com/catalyst/moodle-fileconverter_librelambda/blob/master/classes/converter.php#L202-L206 Therefore the setting page is not displayed at all:

See objectfs as an example: https://github.com/catalyst/moodle-tool_objectfs/blob/master/classes/local/store/s3/client.php#L215-L234

Also as part of IAM role change this line was changed: https://github.com/catalyst/moodle-fileconverter_librelambda/commit/c68fe43702ffa0bf1b0fb13918d52223818f845a#diff-b9a34794852807cf1a15b1525ad6e3dbR106 Thus, proxy is used even when the setting is set to 0.

And the last one: This return isn't expected. Should be bool https://github.com/catalyst/moodle-fileconverter_librelambda/blob/master/classes/converter.php#L323