Use repr to format s3 exceptions

[vin01]

Use of repr would ensure that exceptions without any error message (like MemoryError) are displayed properly to support debugging.

I recently debugged a MemoryError where error message would just say:

download failed: s3://remote to local-path

Having exception information in the log would be very helpful, it is not even displayed in the DEBUG logs currently.

I am not sure if using repr is the best approach here though, wdyt?

[stealthycoin]

This needs an issue outlining the problem, if it gets sufficient traction we could move forward to the implementation phase.
#6828 details our new contribution process.