Better error for semicolon after decorator

[JoshuaKGoldberg]

TypeScript Version: 2.7.0-dev.20171230

Code

import { component } from "babyioc";

import { Member } from "./Effects";

class Example {
    @component(Member);
    public readonly member: Member;
}

Expected behavior:

file.ts(6,23): error TSXXXX: Semicolons not allowed after decorators.

Actual behavior:

file.ts(6,23): error TS1146: Declaration expected.

Not very important IMO, but a little confusing.

[JoshuaKGoldberg]

👋 @RyanCavanaugh I'd still like to tackle this issue but #22013 aged away without a review. Would the team still consider this a help wanted issue? I can resolve merge conflicts and update that PR if so.