Allow exceptions to be logged as ERROR

[fandfisf]

For most projects, a typical structure is

1. Method entry and exist are logged at low severity (DEBUG or INFO)
2. Exceptions are logged as ERROR.

I believe this used to be the default behavior of @Loggable, however issue #149 changed that. I understand the motivations behind it, but for a typical case, this approach of logging everything at the same level does not help. We rarely want to log enter/exit at WARN or exceptions at DEBUG.
With that in mind I am proposing that we allow a flag for @Loggable to log exceptions as "ERROR". I am proposing a solutions and attaching a pull request.
This pull request introduces exceptionsAsError flag for @Loggable. This flag is set to false by default, and hence nothing will change on library upgrade, however one can take advantage of the flag if one wants.

[dmarkov]

@yegor256 please do something about it, see par.21