nonsense help message for stray `=>`

[durka]

Code:

if true => { }

Output:

error: expected `{`, found `=>`
 --> expr.rs:5:10
  |
5 | if true => { }
  |         ^^
  |
  = help: place this code inside a block

"Place this code inside a block" doesn't make any sense here.

[Aatch]

It's there to catch this:

if (foo)
   bar;

But the way it's implemented means that if we get something that isn't a block, we say to put it in a block. We might be able try parsing an expression, and show the help only if that works.