Create a doc for subtensor error messages

[rajkaramchedu]

Create a document for subtensor error messages.

Dev contact: @JohnReedV

Reference gist: https://gist.github.com/JohnReedV/6e0811d220cd8053dc162a8b4e1cd0e1
Reference PR: opentensor/subtensor#700

[rajkaramchedu]

#348