win: set fallback message if FormatMessage fails

FormatMessage can fail, e.g. with ERROR_MUI_FILE_NOT_FOUND. In this case
no error message was set and uv_dlerror wrongly returned "no error".

With this patch the fallback error message "error: <errorno>" is
generated when FormatMessage fails.
win: try fallback to default language in uv_dlerror
tests: check dlerror is "no error" in no error case