Avoid mixing error patterns with error annotations

When both error patterns and error annotations are present in an ui
test, only error patterns are validated against the output.

Replace the error pattern with an error annotation to avoid silently
ignoring the other error annotation.

Author: tmiasko

src/test/ui/continue-after-missing-main.rs

@@ -1,6 +1,4 @@
-#![allow(dead_code)]
-
-// error-pattern:`main` function not found in crate
+#![allow(dead_code)] //~ ERROR `main` function not found in crate
 
 struct Tableau<'a, MP> {
     provider: &'a MP,

src/test/ui/continue-after-missing-main.stderr

@@ -3,15 +3,15 @@ error[E0601]: `main` function not found in crate `continue_after_missing_main`
    |
 LL | / #![allow(dead_code)]
 LL | |
-LL | | // error-pattern:`main` function not found in crate
-LL | |
+LL | | struct Tableau<'a, MP> {
+LL | |     provider: &'a MP,
 ...  |
 LL | |
 LL | | }
    | |_^ consider adding a `main` function to `$DIR/continue-after-missing-main.rs`
 
 error[E0623]: lifetime mismatch
-  --> $DIR/continue-after-missing-main.rs:30:56
+  --> $DIR/continue-after-missing-main.rs:28:56
    |
 LL |     tableau: Tableau<'data_provider, AdaptedMatrixProvider<'original_data, MP>>,
    |              ------------------------------------------------------------------ these two types are declared with different lifetimes...