improve error message when the content of loop is not a block

[arielb1]

This useful-looking fragment

loop match x {
  a => {...}
  b => {...}
}

Gives the confusing error

... error: obsolete syntax: `loop` instead of `continue`
note: `loop` is now only used for loops and `continue` is used for skipping iterations

Which is totally unrelated to the solution - putting a block around loop

loop { match x {
  a => {...}
  b => {...}
}}

Either this kind of construct should be allowed or the error message should be improved.