Fix error from undefined process.stderr

[yehee]

• Fix Uncaught TypeError: Cannot read property 'write' of undefined

[trentm]

@alice-0-kim What environment are you running bunyan in which process.stderr isn't defined? Is this in a browser, perhaps?

(FYI on the delay in my responding: https://github.com/trentm/node-bunyan/issues/335#issuecomment-648583828)

[joshkel]

Instead of checking whether process.stderr is defined, could you replace process.stderr.write(msg + '\n') with console.error(msg)? That should work in a browser as well as Node.js without special handling.