Log as warning instead of exception (#519)
Log as warning instead of exception (#519)

Logging as exception prints a stack backtrace. Since the modules are optional
and listed as extra instead of a required dependency, this should probably be
a warning instead of an error/exception.

Co-authored-by: Lennart Moltrecht <652015cd4e3404c09afab96ad3e1bf8832091929@bmw.com>
ebroecker/canmatrix,ebroecker/canmatrix