Fix BL-15836 Allow Minimal Book constructor for ErrorBook

src/BloomExe/Book/Book.cs

@@ -84,10 +84,19 @@ public class Book
         /// </summary>
         public bool WriteFontFaces = true;
 
-        //for moq'ing only; parameterless ctor required by Moq
+        // For moq'ing only; parameterless ctor required by Moq.
         public Book()
+            : this(false)
         {
-            Guard.Against(!Program.RunningUnitTests, "Only use this ctor for tests!");
+        }
+
+        // Allow specific subclasses (like ErrorBook) to bypass the unit-test guard.
+        protected Book(bool allowNonTestCtor)
+        {
+            if (!allowNonTestCtor)
+            {
+                Guard.Against(!Program.RunningUnitTests, "Only use this ctor for tests!");
+            }
         }
 
         public Book(BookInfo info = null, IBookStorage storage = null)

src/BloomExe/Book/ErrorBook.cs

@@ -14,14 +14,12 @@ public class ErrorBook : Book
         private bool _canDelete;
 
         /// <summary>
-        /// this is a bit of a hack to handle representing a book for which we got an exception while loading the storage... a better architecture wouldn't have this...
+        /// This is a bit of a hack to handle representing a book for which we got an exception while loading the storage.
+        /// A better architecture wouldn't have this.
         /// </summary>
         public ErrorBook(Exception exception, string folderPath, bool canDelete)
+            : base(true)
         {
-            // ENHANCE: Address that a Guard fails when this constructor is called.
-            // This class inherits from Book. So it calls Book's default constructor here.
-            // But Book's default constructor has a Guard that says it's only supposed to be called from the unit tests.
-            // One potential route is to create an interface... IBook or ISimpleBook
             Exception = exception;
             _folderPath = folderPath;
             _canDelete = canDelete;