Correct operation produces log output that is alarming, confusing

[usrlocalben]

Arsenic treats WebDrivers's negative results as Errors.

For example, in many test situations the most likely message traffic
between Arsenic and WebDriver will be "NoSuchElement" as test
implementations sit in wait_for_element() polling loops. These
exceptions are mischaracterized as "errors," resulting in noisy log
output--and confusion.

Logging all exceptions that occur also results in double-logging,
since responses are already logged.

See #122