Don't use OSL_LOG_PREFIX when throwing exceptions in non-debug builds.

Using OSL_LOG_PREFIX in the exceptions is not a good idea; it blows the size
of the string literals tremendously - full build path for every line that uses
that, including the line.

Let's not remove it for good, but hide it for non-debug builds
(OSL_DEBUG_LEVEL == 0) so that anybody who would like to use this during
debugging could still do (but I doubt it has any value for anybody).

Change-Id: Icc8db95ae0862671a206e681f92c60cdf51ffc32