[v2] s3: Handle unlinked current working directory

[stoeckmann]

Issue #, if available:

• https://github.com/aws/aws-cli/issues/7143

Description of changes:

The relative_path function can also fail on Unix-like systems if the current working directory has been unlinked.

Add this except block next to already existing one for Windows systems where different drives might be in use.

Proof of Concept:

1. Get yourself into an unlinked current working directory

MYDIR=$(mktemp -d)
cd $MYDIR
rmdir $MYDIR

2. Try to copy a file with absolute path to an s3 bucket

aws s3 cp /bin/ls s3://bucket

You can see the following output:

fatal error: [Errno 2] No such file or directory

If you analyze the aws call with strace, you can see that actually getcwd fails and that the error message is not about /bin/ls at all:

[pid 56266] getcwd(0x558ff0681790, 1024) = -1 ENOENT (No such file or directory)

With this patch applied, aws continues operation and just prints the absolute path instead of relative one.

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