Metadata - error if file doesn’t exists

We were previously checking for the file’s extistence, but it seems better to error, in order to avoid things like:

#8423 (reply in thread)

Author: dragonstyle

src/config/metadata.ts

@@ -80,7 +80,9 @@ export async function includedMetadata(
         throw e;
       }
     } else {
-      return undefined;
+      throw new Error(
+        `The \`metadata-file\` ${yamlFile} doesn't exist. Please check that the path to the file is correct.`,
+      );
     }
   })) as Array<Metadata>;
 
@@ -220,7 +222,7 @@ export function metadataAsFormat(metadata: Metadata): Format {
 
   // coalese ipynb-filter to ipynb-filters
   const filter = format.execute[kIpynbFilter];
-  if (typeof (filter) === "string") {
+  if (typeof filter === "string") {
     typedFormat.execute[kIpynbFilters] = typedFormat.execute[kIpynbFilters] ||
       [];
     typedFormat.execute[kIpynbFilters]?.push(filter);
@@ -302,7 +304,7 @@ export function mergeProjectMetadata<T>(
   return mergeConfigsCustomized<T>(
     (objValue: unknown, srcValue: unknown, key: string) => {
       if (
-        kExandableStringKeys.includes(key) && typeof (objValue) === "string"
+        kExandableStringKeys.includes(key) && typeof objValue === "string"
       ) {
         return srcValue;
       } else {
@@ -361,7 +363,7 @@ export function mergeDisablableArray(objValue: unknown, srcValue: unknown) {
 
 export function mergePandocVariant(objValue: unknown, srcValue: unknown) {
   if (
-    typeof (objValue) === "string" && typeof (srcValue) === "string" &&
+    typeof objValue === "string" && typeof srcValue === "string" &&
     (objValue !== srcValue)
   ) {
     // merge srcValue into objValue