buf convert error messages should be more clear when type does not exist in input

[joshcarp]

Expected:

buf convert . --from=payload.json --to=- --type=Person          
Failure: proto: not found

Actual

buf convert . --from=payload.json --to=- --type=Person          
Failure: type Person not found in input

[bufdev]

@saquibmian this is low priority, no need for this for now

[doriable]

Closing this for now since this message is coming from protoreflect and is currently an upstream concern. In the future, we may invest in cleaning up messages from upstream packages, but that should be captured in a broader effort.