VS: Verify toolset version= field format more strictly

In commit 5f13168419 (VS: Add option to select the version of the
toolset used by VS 2017, 2018-05-19, v3.12.0-rc1~38^2) we added logic to
verify that the toolset version, such as `14.35`, matches the toolset
name, such as `v143`.  Clarify the logic to not construct a temporary
nonsensical toolset name like `v1435`.  Also verify the format of the
toolset version more strictly, e.g., to reject `14.350` earlier.
Previously the latter example was only rejected by the `.props` file not
existing.