ethexe(rpc): return standard json rpc error codes

[techraed]

Problem to Solve

Error code used right now is 8000 which is valid, but not standard.

Possible Solution

Replace with standard codes for application/server errors (from -32099 to -32000)

[breathx]

8000 is actively used in substrate so its generally ok, but i'd insert more information everywhrere in rpc errors