fix: add validation for uploaded results, handle errors

[Shelex]

Rationale

Currently it is impossible to debug the report generation as few errors were handled during uploading.

Added

• validate result file buffer which should be a zip file with pw-specific folder and file
• add result file buffer validation to /api/result/upload endpoint
• add more error handling to report index.html parser
• show any stderr from playwright report generation
• add more error handling to fs storage saving results