Keep /EHsc when clearing the MSVC default flags (#7185)

The MSVC block drops CMake's defaults wholesale, and /EHsc goes with them, so
Windows builds compile every try/catch without unwind semantics - cl only warns
(C4530). soh and libultraship have 25 of them between them, including the
config and archive paths in OTRGlobals, where a throw would skip destructors.

clang-cl refuses outright rather than warning, which is how this surfaced.

soh/CMakeLists.txt

@@ -59,7 +59,8 @@ if(MSVC)
     set(CMAKE_C_FLAGS "")
     set(CMAKE_C_FLAGS_DEBUG "")
     set(CMAKE_C_FLAGS_RELEASE "")
-    set(CMAKE_CXX_FLAGS "")
+    # Keep /EHsc: the code uses try/catch.
+    set(CMAKE_CXX_FLAGS "/EHsc")
     set(CMAKE_CXX_FLAGS_DEBUG "")
     set(CMAKE_CXX_FLAGS_RELEASE "")
 endif()