Make cfail test error messages more precise

Closes rust-lang#3192

Author: alexcrichton

src/test/compile-fail/struct-no-fields-2.rs

@@ -8,11 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern: Unit-like struct construction is written with no trailing `{ }`
 struct Foo;
 
 fn f2() {
     let _end_stmt     = Foo { };
+    //~^ ERROR: Unit-like struct construction is written with no trailing `{ }`
 }
 
 fn main() {}

src/test/compile-fail/struct-no-fields-3.rs

@@ -8,11 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern: Unit-like struct construction is written with no trailing `{ }`
 struct Foo;
 
 fn g3() {
     let _mid_tuple    = (Foo { }, 2);
+    //~^ ERROR: Unit-like struct construction is written with no trailing `{ }`
 }
 
 fn main() {}

src/test/compile-fail/struct-no-fields-4.rs

@@ -8,11 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern: Unit-like struct construction is written with no trailing `{ }`
 struct Foo;
 
 fn h4() {
     let _end_of_tuple = (3, Foo { });
+    //~^ ERROR: Unit-like struct construction is written with no trailing `{ }`
 }
 
 fn main() {}

src/test/compile-fail/struct-no-fields-5.rs

@@ -8,11 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern: Unit-like struct construction is written with no trailing `{ }`
 struct Foo;
 
 fn i5() {
     let _end_of_block = { Foo { } };
+    //~^ ERROR: Unit-like struct construction is written with no trailing `{ }`
 }
 
 fn main() {}

src/test/compile-fail/struct-no-fields.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern: Unit-like struct definition should be written as `struct Foo;`
 struct Foo {}
+//~^ ERROR: Unit-like struct definition should be written as `struct Foo;`
 
 fn main() {}