Reduce likelihood of shadowing symbols in OPM_THROW

The OPM_THROW macro uses an internal symbol, `oss`, which is, strictly
speaking, in the client's namespace.  Moreover, as the
`EclipseGridParseHelpers.hpp` header uses the same symbol, and for
similar purposes, this produces warnings at `-Wshadow` in GCC when
compiling that header.

Rename the macro's symbol to `oss__` (double trailing underscore) to
reduce the likelihood of shadowing a previous definition.