feat(Base): Add region when scan exception occures

[acolombier]

Hi PynamoDB team!

First, thanks for your great work!

This PR comes after a painful debug session to catch a "table not found" issue. In the end, it turned out that the region had been change in our config file.

A few day ago, the issue occurred again, and even if this time, it took me only a few minutes to remember this issue, I thought it would be great to display the region which is currently used for scanning, and then help people catching a potential region misconfiguration, especially since the region is defaulted in settings.py.

Hope you like the idea,
Thanks

[ikonst]

I'm not sure what this exception normally says (does it have the table's name?) but I think this change would be overly specific to the problem you faced -- "fighting the last war", so to speak.

[acolombier]

Hi @ikonst ,

Sorry, I should have stated that first.

Basically the error message from boto3 looks like the following:

An error occurred (ResourceNotFoundException) when calling the <OperationName> operation: Requested resource not found: Table: <TableName> not found

boto3 wouldn't display the region because it is a mandatory setting (if you don't set it up in the code, it is expecting it to find it in your profile definition). Here is what happen if boto3 cannot resolve an explicitly set region:

botocore.exceptions.NoRegionError: You must specify a region.

However, PynamoDB is "breaking" this safeguard by explicitly defaulting the region to us-east-1 and thus, brings to confusion when you are used to AWS services(/boto3) requirements.

If you wish, the aim of this PR is to bring the safeguard in a non-destructive way. :)

I hope that makes it more clear.

Thanks.