Add new error code

src/test/ui/missing-items/m2.stderr

@@ -1,4 +1,4 @@
-error: main function not found
+error[E0601]: main function not found
 
 error[E0046]: not all trait items implemented, missing: `CONSTANT`, `Type`, `method`
   --> $DIR/m2.rs:20:1

src/test/ui/resolve/issue-14254.stderr

@@ -142,7 +142,7 @@ error[E0425]: cannot find value `bah` in this scope
 133 |         bah;
     |         ^^^ did you mean `Self::bah`?
 
-error: main function not found
+error[E0601]: main function not found
 
 error: aborting due to previous error(s)
 

src/test/ui/resolve/issue-21221-2.stderr

@@ -7,7 +7,7 @@ error[E0405]: cannot find trait `T` in this scope
 help: possible candidate is found in another module, you can import it into scope
    | use foo::bar::T;
 
-error: main function not found
+error[E0601]: main function not found
 
 error: cannot continue compilation due to previous error
 

src/test/ui/resolve/suggest-path-instead-of-mod-dot-item.stderr

@@ -72,7 +72,7 @@ error[E0423]: expected function, found module `a::b`
    |        |
    |        did you mean `I`?
 
-error: main function not found
+error[E0601]: main function not found
 
 error: aborting due to previous error(s)
 

src/test/ui/span/issue-35987.stderr

@@ -7,7 +7,7 @@ error[E0404]: expected trait, found type parameter `Add`
 help: possible better candidate is found in another module, you can import it into scope
    | use std::ops::Add;
 
-error: main function not found
+error[E0601]: main function not found
 
 error: cannot continue compilation due to previous error
 

src/test/ui/token/issue-10636-2.stderr

@@ -22,7 +22,7 @@ error: expected expression, found `)`
 19 | } //~ ERROR: incorrect close delimiter
    | ^
 
-error: main function not found
+error[E0601]: main function not found
 
 error: aborting due to previous error(s)
 

src/test/ui/token/issue-41155.stderr

@@ -12,7 +12,7 @@ error[E0412]: cannot find type `S` in this scope
 11 | impl S {
    |      ^ not found in this scope
 
-error: main function not found
+error[E0601]: main function not found
 
 error: aborting due to previous error(s)