MessageEvent is not defined in Node, breaking streaming.

[abuiles]

Describe the bug
The onerror function defined in .stream checks if the received error is an instance of MessasgeEvent - this works fine in the browser but it will fail if you are running in node, since MessageEvent is not part of Node's API.

js-stellar-sdk/src/call_builder.ts

Line 144 in 992e828

if (options.onerror && error instanceof MessageEvent) {

What version are you on?
2.2.0

To Reproduce
Steps to reproduce the behavior:

1. git clone js-stellar-canary
2. cd js-stellar-canary
3. yarn
4. node --experimental-modules src/streaming.js

Expected behavior
If there is an error from the stream, then it should be handled by my onerror function, but instead we get

/js-stellar-canary/node_modules/stellar-sdk/lib/call_builder.js:75
                    if (options.onerror && error instanceof MessageEvent) {
                                                            ^

ReferenceError: MessageEvent is not defined
    at EventSource.es.onerror (/js-stellar-canary/node_modules/stellar-sdk/lib/call_builder.js:75:61)
    at EventSource.emit (events.js:203:13)
    at _emit (/js-stellar-canary/node_modules/eventsource/lib/eventsource.js:242:17)
    at onConnectionClosed (/js-stellar-canary/node_modules/eventsource/lib/eventsource.js:52:5)
    at IncomingMessage.<anonymous> (/js-stellar-canary/node_modules/eventsource/lib/eventsource.js:169:9)
    at IncomingMessage.emit (events.js:208:15)
    at endReadableNT (_stream_readable.js:1154:12)
    at processTicksAndRejections (internal/process/task_queues.js:77:11)

Additional context
This error was originally reported by @danieldeusing on keybase