Rename EndpointError to ErrorDetail

[andrew-coleman]

The gateway connects to and invokes external gRPC endpoints and if any of those return an error, they get translated into EndpointError proto messages and sent back to the client in the Details section of the google.rpc.Status error message.
Depending on the SDK language, this will get embedded in the resultant client exception. The current name, ending in 'Error' has unintended connotations when used in Node and Java, since this is not an exception or error in its own right.

This issue is to rename the structure 'ErrorDetail'.
Changes need to be made to (in order):
fabric-protos: /gateway/gateway.proto
fabric: The gateway component that builds and marshals this message
fabric-gateway: The Node SDK that unmarshals this structure, and the samples for the 3 SDKs.