fix: serving STFC PDFs from Postgres

[simonfernandes]

Refs UserOfficeProject/issue-tracker#1163

• Fix the postgres error we're seeing for invalid large-object descriptor
  • The problematic part was proposalPdfData.emit('end'); as proposalPdfData.pipe(res); has already closed the stream at that point
• Refactor the db retrieval to add more robust error handling
• Add logging so that we can see when a proposal was downloaded via postgres and whether it was successful or not