Better error messages when note using async features correctly (T7214)

[babel-bot]

Issue originally made by @andresn

Options

Can we have something more user friendly other than syntax error, unexpected token under these kinds of scenarios:

function main() {
    var quote = await this.getQuote(); // only produces "Syntax error... unexpected token"
    console.log(quote);
}

/\

the correct way (if you didn't catch it):

async function main() {
     var quote = await this.getQuote(); // something like: "use of await method without parent method marked async"
    console.log(quote);
}

Description

[motiz88]

Closing in favor of babel/babylon#113.