Convert err obj into JSON stringifiable object + update error msg

[vicky-g]

Description

What is the purpose of this PR? What is the current behavior? New behavior? Relevant links (e.g. Trello) and/or information pertaining to PR?

Context: We've been seeing error messages that look like [Object object].
Purpose:

• This fix is to serialize the error into an object that JSON.stringify() can parse.
• Also updates the error msg for comparing dir CIDs to include the expected CID

Tests

List the manual tests and repro instructions to verify that this PR works as anticipated. Include log analysis if possible.
❗ If this change impacts clients, make sure that you have tested the clients ❗

Ran mad-dog to encounter error

Before:

Network Error in request 8e033b4c-e336-4c67-8f40-f3ad0d22b080 with 1 retries... retrying
Error: [object Object]
    at CreatorNode._uploadFile (/Users/vickyguan/Documents/audius/audius-protocol/libs/src/services/creatorNode/index.js:688:15)
    at process._tickCallback (internal/process/next_tick.js:68:7)

After:

Network Error in request c0acdb1b-9bdd-4481-b9f0-2da5960654f1 with 1 retries... retrying
Error: {"stack":"Error: Image file validation failed - dirCIDs do not match for dirCID=QmTygvnm5QjCxjr53Bgh6aSwvZUQumFFkF6m9NJD6ZV39i expectedCID=QmPhzRHfy3KGaErjMr8mAfaTcMLfZNet1cCUv5AVtdaFw4\n    at app.post.handleResponseWithHeartbeat (/usr/src/app/src/routes/files.js:414:13)\n    at process._tickCallback (internal/process/next_tick.js:68:7)","message":"Image file validation failed - dirCIDs do not match for dirCID=QmTygvnm5QjCxjr53Bgh6aSwvZUQumFFkF6m9NJD6ZV39i expectedCID=QmPhzRHfy3KGaErjMr8mAfaTcMLfZNet1cCUv5AVtdaFw4"}
    at CreatorNode._uploadFile (/Users/vickyguan/Documents/audius/audius-protocol/libs/src/services/creatorNode/index.js:688:15)
    at process._tickCallback (internal/process/next_tick.js:68:7)

[SidSethi]

love this 💯