fix: Handle FileNotInStorage error when building report

[scott-codecov]

Seems like some very old commits can be missing the chunks file in GCS. We can't really reconstruct a Report instance in that case so we'll just return None (this is similar to worker behavior found here: https://github.com/codecov/worker/blob/d16beed689f0298b7a0fc14a84f06edaae9778a6/services/report/__init__.py#L435)

Fixes codecov/engineering-team#128

[codecov-staging]

Codecov Report

Patch coverage is 66.66% of modified lines.

Files Changed	Coverage
services/report.py	66.66%

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[codecov]

Codecov Report

Merging #81 (12b7a58) into main (d576544) will increase coverage by 0.04%.
The diff coverage is 66.66%.

❗ Current head 12b7a58 differs from pull request most recent head 67313db. Consider uploading reports for the commit 67313db to get more accurate results

@@           Coverage Diff           @@
##            main     #81     +/-   ##
=======================================
+ Coverage   95.25   95.29   +0.04     
=======================================
  Files        695     694      -1     
  Lines      14769   14748     -21     
=======================================
- Hits       14068   14053     -15     
+ Misses       701     695      -6     

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Files Changed	Coverage Δ
services/report.py	92.30% <66.66%> (-2.94%)	⬇️

... and 5 files with indirect coverage changes