Merge #142

142: attributes: turn panics into compile errors r=therealprof a=japaric

Sample error messages:

```
error: `#[entry]` function must have signature `[unsafe] fn() -> !`
  --> examples/error.rs:15:1
   |
15 | fn main() {
   | ^^

error: aborting due to previous error
```

```
error: This attribute accepts no arguments
  --> examples/error.rs:14:1
   |
14 | #[entry(hello)]
   | ^^^^^^^^^^^^^^^

error: aborting due to previous error
```

```
error: This is not a valid exception name
  --> examples/error.rs:20:4
   |
20 | fn foo() {}
   |    ^^^

error: aborting due to previous error
```

Co-authored-by: Jorge Aparicio <jorge@japaric.io>

1 files changed, 2 insertions, 2 deletions

diff --git a/cortex-m-rt/tests/compile-fail/exception-bad-signature-2.rs b/cortex-m-rt/tests/compile-fail/exception-bad-signature-2.rs
index ed46cf6..d2fb210 100644
--- a/cortex-m-rt/tests/compile-fail/exception-bad-signature-2.rs
+++ b/cortex-m-rt/tests/compile-fail/exception-bad-signature-2.rs
@@ -11,8 +11,8 @@ fn foo() -> ! {
     loop {}
 }
 
-#[exception] //~ ERROR custom attribute panicked
-//~^ HELP `#[exception]` functions other than `DefaultHandler` and `HardFault` must have signature `[unsafe] fn() [-> !]`
+#[exception]
 fn SysTick() -> u32 {
+    //~^ ERROR `#[exception]` handlers other than `DefaultHandler` and `HardFault` must have signature `[unsafe] fn() [-> !]`
     0
 }