OasisDataManager Refactor

[sambles]

Issue Description

Tidy up is needed for the PR OasisLMF/OasisLMF#1441

• Update an simplify the OasisDataManager interface, for example shorten class oasis_data_manager.filestore.backends.aws_storage.AwsObjectStore -> oasis_data_manager.filestore.aws or
  oasis_data_manager.df_reader.reader.OasisPandasReader -> oasis_data_manager.PandasReader

• Separate Exceptions between packages
  https://github.com/OasisLMF/OasisLMF/pull/1441/files/62b482a5eeadc2c7ec57e29a6cd583f1272c9321#diff-c724eb010649edb521fe1d00bb3a88fd528b9376df8c5e0e033d7d0e89cf3994

• Clean up exposure file filter names, rename location to location_filter and update description
  https://github.com/OasisLMF/OasisLMF/blob/2d61deffaafb041c1405585e924e1717031a59fd/oasislmf/computation/generate/files.py#L105-L107

https://github.com/OasisLMF/OasisLMF/blob/2d61deffaafb041c1405585e924e1717031a59fd/oasislmf/computation/generate/files.py#L134-L136

• Expand testing to fix drop in code coverage 82.80% -> 75.22%
  https://app.codecov.io/gh/OasisLMF/OasisLMF/pull/1441?src=pr&el=h1&utm_medium=referral&utm_source=github&utm_content=checks&utm_campaign=pr+comments&utm_term=OasisLMF