Unify fallback and unimplemented API error handling

- Add a shared common error response schema and unknown error code for business-level fallbacks
- Return business error payloads for router NoRoute and NoMethod fallbacks
- Align the existing event list fallback response with the shared error structure
- Introduce an explicit unimplemented API error type instead of relying on formatted strings
- Make /api batch dispatch return per-item status 600 error entries for unimplemented modules and actions without failing the whole batch

Signed-off-by: Sean Du <do4suki@gmail.com>

internal/constant/errorcode.go

@@ -4,6 +4,8 @@ type ErrorCode int
 
 // https://github.com/DarkEnergyProcessor/NPPS4/blob/master/npps4/idol/error.py
 const (
+	ErrorCodeUnknown                 ErrorCode = 1
 	ErrorCodeNoUnitIsSellable        ErrorCode = 1205
 	ErrorCodeLivePreciseListNotFound ErrorCode = 3421
+	ErrorCodeEventNoEventData        ErrorCode = 10004
 )