Correctly bubble up rejected promises from TeamSync

[jaller94]

Floating promises could lead to uncaught errors in TeamSync

Failing user or channel syncs are currently resulting in an uncaught promise. That a deprecated functionality of Node, which will crash the end the process soon anyway and would lead to inconsistent behaviour.

.setAvatarUrl threw because Synapse doesn't like undefined

Also, .setAvatarUrl requires a string. The Element client resets the avatar by providing an empty array, so this fixes #453.

Broken counter for creating team clients

const teams = [,,];
const promiseFunctions = [];
let i = 0;
for (const team of teams) {
    i++;
    promiseFunctions.push(async() => {
        console.log(`${i} of ${teams.length}`);
    });
}
promiseFunctions.forEach(func => func());

outputs

3 of 3
3 of 3
3 of 3

because at the time the promises execute, the variable i from the outer context already has its final value 3.