Log full OS error message when the connection fails

[barnabasdomozi]

It can sometimes happen that an OSError object's strerror attribute is None instead of a proper error message.
Example of this behavior:

[INFO] - Get missing file content hashes from the server...
[ERROR] - Connection failed.
[ERROR] - None
[ERROR] - Check if your CodeChecker server is running.

Instead of printing strerror, we should log the full error message.