This is a bug in the .NET Core Tools for VS2015 installer.
I got this error when I tried to uninstall all VS2015 related tools on Windows 10 (not reinstallation, because I want VS2017 instead). I uninstalled VS2015 before removing .NET Core Tools. While trying to remove .NET Core Tools, it tried to call WebToolsExtensionsVS14 for repair; WebToolsExtensionsVS14 failed because VS2015 no longer existed.
I found the MSI package for WebToolsExtensionsV14 in the package cache:
C:\ProgramData\Package
Cache\{BFC344B0-DCDE-36EB-AB05-CB194F2BE946}v14.1.21111.0\packages\WebToolsExtensionsVS14_69.msi
and right-click uninstalled it. Then the .NET Core Tools uninstaller worked.
My recommendation is to remove core tools before removing VS itself.
I found some related issue here, after I figured it out. https://social.msdn.microsoft.com/Forums/vstudio/en-US/ae380a17-60e0-43a4-a481-6e58915e7897/net-core-errors-on-installation-webtoolsextensionsvs14-encountered-error-0x80070643?forum=vssetup