server: Tweak server error string format
server: Tweak server error string format

This just tweaks the error string format introduced in #1530 to be
consistent with other error strings in OPA (e.g., topdown, ast, etc.)

Signed-off-by: Torin Sandall <933af5bec0bc58056526d55ca6c7b45fcf14cc48@gmail.com>

open-policy-agent/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa