This issue has now been resolved in KB4100403.
To get the Windows 10 April 2018 Update (version 1803), we encourage you to wait until the update is offered to your device. If you’re an advanced user on an actively serviced version of Windows 10 and would like to install the Windows 10 April 2018 update now, you can manually check for updates. In the Search tab, type “Settings”. On the Settings page, navigate to Update & security > Windows Update > click “**Check for updates”,**which **** will include this resolution.
I wouldn't recommend attempting the update at all at this stage, even with this "fix". It seems KB4100403 is a fix ONCE 1803 is installed, which, if you have an Intel SSD (I have a brand new Surface Pro) means you will end up with issues before you can get a chance to get KB4100403. Surely you would have pushed that KB4100403 into the 1803 in some fashion and/or a preliminary patch to avoid errors getting 1803 installed?
My experience over the past 2 weeks- https://answers.microsoft.com/en-us/surface/forum/surfpronew-surfupdate/windows-10-version-1803-wont-install-on-new/59d85a0c-a106-4616-a125-ade69903ed95