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.
Merge pull request #365 from bska/shadowing-symbols

Reduce likelihood of shadowing symbols in OPM_THROW