Do not throw exception if file exists

Version from pip3 - 2.1.0 - crashes when a file already exists in the bucket:

```
(s4cmd) $ s4cmd put bla.zip s3://bla/bla/
[Runtime Failure] File already exists: s3://bla/bla/bla.zip
Exception in thread Thread-1:ead(s)]
Traceback (most recent call last):
[Thread Failure] File already exists: s3://bla/bla/bla.zip
```

I wonder if an exception is unavoidable or if s3 can return an error to evaluate. I also think the exception needs to be caught and treated.

Start by reproducing the `s4cmd put bla.zip s3://bla/bla/` command against a bucket containing the target file, then trace the resulting runtime and thread failures. Done means an existing destination is handled without an uncaught exception, with behavior consistent with the intended S3 error response.

Written by the indexing model from the issue text.