To answer my own question, and this applies to all three platforms (WIN, OSX, LNX), reason for this error is that a folder is missing from the Required files for the latest master causing the compile to fail. You will not be able to compile the Editor without it I’m afraid.
Engine/Content/Internationalization is required, but because it’s missing, UHT crashes not finding I18N files, causing UBT to return -1.
This cryptic message, and return code, is really not helpful and I hope someone at Epic takes a look and decides to make it more verbose.
Starting with 4.6 (as well as the master) the previous required zip files are no longer shown along with the source zip. After downloading and extracting the source zip file to your computer you’ll want to run the Setup.bat which includes the required files. After that runs you need to then run the GeneratedProjectFiles.bat to create the Visual Studios .sln file. You then only need to open and build the .sln file to build the engine.