Improve error when missing macro argument

[dtolnay]

The log! macro matches part of its arguments as $($arg:tt)+, but the other macros match it as $($arg:tt)*. That means if a user calls error!() or any of the other macros with an empty argument, it successfully matches the macro rules of the first macro they called but then turns into an invalid call to log!, leading to a confusing error message.

This PR uses $($arg:tt)+ consistently to match arguments.

Example:

use log::error;

fn main() {
    error!();
}

Before:

error: expected identifier, found `,`
 --> src/main.rs:4:5
  |
4 |     error!();
  |     ^^^^^^^^^
  |     |
  |     expected identifier
  |     help: remove this comma
  |
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

After:

error: unexpected end of macro invocation
 --> src/main.rs:4:5
  |
4 |     error!();
  |     ^^^^^^^^^ missing tokens in macro arguments

Closes #271 (comment).