Standard library functions that are presented with an input for which they are undefined call the standard library function Fail with the name of the calling standard library function prefixed to the input. The standard library function Fail is defined essentially as follows:
Fail: <$x> == 'Error:' $x failed on path '<' Idem:<> '>'.
Note that the actual implementation of Fail should format the path conventionally - without initial and final spaces. As with all other standard library functions, Fail is only defined as such if it is explicitly invoked with a uses directive. The user may thus choose (i) to invoke it, (ii) not to invoke it but to define it however they wish, or (iii) to simply leave it undefined.
Like all other standard library functions, Fail can be renamed. But you are unlikely to want to rename it since it will serve no useful purpose under another name (the standard library functions invoke it by its original name).