Set connectionFailed to Print error log by default.

[DHosseiny]

Desirable behavior is to see error log when something unexpectedly happened.

[PHELAT]

Thank you for your contribution on this project.
It would be much nicer if user could turn off this default behavior using a flag, perhaps inside PaymentConfiguration class you could add a flag for this purpose. Also, it's much preferred to have this behavior throughout the library and not just for connectionFailed callback.