Interpolate the log message also for logged exceptions

According to PSR-3 the log message can have placeholders that are
replaced from the context object. Our logger implementation did that for
all PSR-3 logger methods. The only exception was our custom `logException`.

Since PsrLoggerAdapter calls logException when an exception key is
present in the context object, log messages were no longer interpolated.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>