Expression to describe error’s way

Fallback route and detour or detour route seem like good suggestions, but you might also consider rerouting, in the latter sense of its definition “The process by which something is rerouted; a diversion or redirection”. (Redirection, “the automated process of taking a user to a location other than the one selected”, slightly misses the point, as it refers to the rerouting process rather than the rerouting itself, but redirecting might work; as in “The redirecting is shown here”.) The slightly shorter forms reroute and redirect might also be used as names of a substituted route.

Some related terms – none of which are quite suitable, but may offer food for thought – are failover (“An automatic switch to a secondary system on failure of the primary system”), back channel (“An unofficial communications channel used to make informal or subversive negotiations”), provenance (“(computing) The copy history of a piece of data, or the intermediate pieces of data utilized to compute a final data element, as in a database record or web site (data provenance)” or “(computing) The execution history of computer processes which were utilized to compute a final piece of data (process provenance)”), traveller (“A list and record of instructions that follows a part in a manufacturing process”), and waybill (“A document that lists the final destination (and other details) of each part of a cargo”).

From a networking perspective, route is used to describe the course that is taken to reach the destination. It is used both as a noun and as a verb (to describe the process of getting to the destination). Using this terminology, an appropriate term for the "redirected path" in case of a failure would be fallback route.

These may be exclusive to American English, and they build on the use of the noun route suggested by coleopterist: alternate route or detour route.

I think of both terms referring to traffic, but they can be used in other contexts without ambiguity in American English.