Fix version check script (#25)

The Version check script started failing on master, but it was also
crashing on exit, losing the error message. This fixes the exit code, in
the way described in the Node.js documentation
[here](https://nodejs.org/api/process.html#processexitcode)

tools/verify-rust-version.js

@@ -18,8 +18,6 @@ const cargo_toml_rust_version = cargo_toml_src.split('\n').filter(l => l.startsW
 
 console.log(`Rust version: ${cargo_toml_rust_version}`);
 
-let error = false;
-
 for (const file of files) {
     const file_content = fs.readFileSync(file, 'UTF8').toString();
     const matches1 = file_content.match(re1);
@@ -29,7 +27,7 @@ for (const file of files) {
         for (const match of matches1) {
             if (match !== cargo_toml_rust_version.substring(0, 4)) {
                 console.error(`Found reference to version ${match}, expected ${cargo_toml_rust_version} in file ${file}`);
-                error = true;
+                proc.exitCode = 1; // Non zero code indicates error
             }
         }
     }
@@ -38,11 +36,9 @@ for (const file of files) {
         for (const match of matches2) {
             if (match !== cargo_toml_rust_version) {
                 console.error(`Found reference to version ${match}, expected ${cargo_toml_rust_version} in ${file}`);
-                error = true;
+                proc.exitCode = 1; // Non zero code indicates error
             }
         }
     }
 
 }
-
-proc.exit(error);