Add wxMSWFormatMessage() and use it from other places

Don't duplicate calls to ::FormatMessage(), which is difficult to use
correctly, in wxCrashReport and wxWebRequestWinHTTP, but just reuse the
same code that was already present in wxSysErrorMsgStr() after
refactoring it into a reusable function allowing to specify the module
name to use for the error code lookup (before falling back to
interpreting it as system error code).

This fixes not trimming the trailing "\r\n" from the string in the other
places (wxWinHTTPErrorToString() had code to do it, but it was wrong,
while wxCrashContext::GetExceptionString() didn't do it at all) and
avoids duplication.

include/wx/msw/private.h

@@ -972,6 +972,10 @@ enum wxWinVersion
 
 WXDLLIMPEXP_BASE wxWinVersion wxGetWinVersion();
 
+// This is similar to wxSysErrorMsgStr(), but takes an extra HMODULE parameter
+// specific to wxMSW.
+WXDLLIMPEXP_BASE wxString wxMSWFormatMessage(DWORD nErrCode, HMODULE hModule = 0);
+
 #if wxUSE_GUI && defined(__WXMSW__)
 
 // cursor stuff