ConfigException if the specified bucket doesn't exist

[muga]

If the specified bucket doesn't exist, S3 input plugin should throw ConfigException because this error happens by user's wrong configuration. In the current implemention, AmazonS3Exception is directly thrown.

2015-11-21 04:33:54.283 +0000 [WARN] (main): The specified bucket does not exist (Service: Amazon S3; Status Code: 404; Error Code: NoSuchBucket; Request ID: 3BE58F8F886BD369)
com.amazonaws.services.s3.model.AmazonS3Exception: The specified bucket does not exist (Service: Amazon S3; Status Code: 404; Error Code: NoSuchBucket; Request ID: 3BE58F8F886BD369)
    at com.amazonaws.http.AmazonHttpClient.handleErrorResponse(AmazonHttpClient.java:1239) ~[aws-java-sdk-1.5.2.jar:na]
    at com.amazonaws.http.AmazonHttpClient.executeOneRequest(AmazonHttpClient.java:823) ~[aws-java-sdk-1.5.2.jar:na]
    at com.amazonaws.http.AmazonHttpClient.executeHelper(AmazonHttpClient.java:506) ~[aws-java-sdk-1.5.2.jar:na]
    at com.amazonaws.http.AmazonHttpClient.execute(AmazonHttpClient.java:318) ~[aws-java-sdk-1.5.2.jar:na]
    at com.amazonaws.services.s3.AmazonS3Client.invoke(AmazonS3Client.java:3595) ~[aws-java-sdk-1.5.2.jar:na]
    at com.amazonaws.services.s3.AmazonS3Client.invoke(AmazonS3Client.java:3548) ~[aws-java-sdk-1.5.2.jar:na]
    at com.amazonaws.services.s3.AmazonS3Client.listObjects(AmazonS3Client.java:647) ~[aws-java-sdk-1.5.2.jar:na]
    at org.embulk.input.s3.AbstractS3FileInputPlugin.listS3FilesByPrefix(AbstractS3FileInputPlugin.java:167) ~[na:na]
    at org.embulk.input.s3.AbstractS3FileInputPlugin.listFiles(AbstractS3FileInputPlugin.java:150) ~[na:na]
    at org.embulk.input.s3.AbstractS3FileInputPlugin.transaction(AbstractS3FileInputPlugin.java:83) ~[na:na]
    at org.embulk.spi.FileInputRunner.transaction(FileInputRunner.java:65) ~[embulk-core-0.7.7.jar:na]

[frsyuki]

I think this implementation is unnecessarily verbose. I created #21 instead.

[frsyuki]

#20 requires another permission by the way (ListBuckets against AWS account, in addition to ListObjects/GetObject against a bucket). So it breaks backward compatibility.