How to exclude lcov branches within a macro

the new lcov version 1.11 (or 1.12) introduce LCOV_EXCL_BR_LINE keyword. So in your case:

LOG_MSG(ERROR, "An Error has occurred\n"); //LCOV_EXCL_BR_LINE

or, even better:

LOG_MSG(ERROR, "An Error has occurred\n"); (void)("LCOV_EXCL_BR_LINE");

which survives precompiler comment stripping.