There is no option for relative line numbers. I have following ideas:
Go to start of the routine with the error, press Ctrl+Shift+Home to select everything to top of the file and then Ctrl+X to cut it. Now goto the line with the error and correct it, Press Ctrl+Home and then paste back with Ctrl+V the top of the file.
Go to start of the routine with the error, activate the column mode with Alt+C, press key Shift and hold it why you move the cursor down with key DOWN ARROW to end of the routine. Press now the hotkey for Column - Insert Number to insert in the select lines an increasing number starting with 0. Go to the line with the error, press Ctrl+Z to undo the inserting of the numbers and press Alt+C to switch back to normal edit mode and now you can correct the error.
Last suggestion would be what I would do. Assuming the ouput of the programmer is captured into the output window or a file I would write a script which copies the text in the output window or file and reformats it to the format required by UE/UES to simply double click on a line with an error in the output window to open the file (if not already open) and position the cursor on the line with the error. With a script it is no problem to calculate always the absolute line number of an error. That is definitely the a better solution as 1st and 2nd suggestion but requires some programming skills.
A fourth method would be the best solution, but would require much more programming skills. I have written for myself years ago 2 programs which filter the compiler outputs of "Turbo C" (very old DOS version) and Tasking C166 compiler/assembler/linker directly during compilation of a project to get the output UE/UES need. That lets me run the compilers directly from within UE/UES and I get the errors in the output ready for easy reading it, navigation to it and correct it. If you write something similar for MUMPS, you would need no extra step inside UEStudio to go to an error and correct it.
Best regards from Austria