DI-930 fix #198 - allow invalid samples in manifest when subsetting

[jethror1]

• Fixes #198
• When an invalid sample identifier / missing is present in the manifest, this would normally raise an error on parsing
• This will now not raise an error if -isubset is provided, and all sample IDs provided to -isubset are valid

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This change is 

[pep8speaks]

Hello @jethror1! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file resources/home/dnanexus/dias_batch/tests/test_utils.py:

Line 809:5: E303 too many blank lines (2) Line 830:5: E303 too many blank lines (2)