refactor: throw `NotRegisteredException` if API class provider is bad

bukkit/src/main/java/net/william278/husksync/api/BukkitHuskSyncAPI.java

@@ -28,6 +28,7 @@ import net.william278.husksync.user.OnlineUser;
 import net.william278.husksync.user.User;
 import org.bukkit.entity.Player;
 import org.bukkit.inventory.ItemStack;
+import org.bukkit.plugin.java.JavaPlugin;
 import org.jetbrains.annotations.ApiStatus;
 import org.jetbrains.annotations.NotNull;
 
@@ -59,6 +60,10 @@ public class BukkitHuskSyncAPI extends HuskSyncAPI {
      */
     @NotNull
     public static BukkitHuskSyncAPI getInstance() {
+        if (!JavaPlugin.getProvidingPlugin(BukkitHuskSyncAPI.class).getName().equals("HuskSync")) {
+            throw new NotRegisteredException("This is likely because you have shaded HuskSync into your plugin JAR " +
+                    "and need to fix your maven/gradle/build script so that it *compiles against* HuskSync instead.");
+        }
         if (instance == null) {
             throw new NotRegisteredException();
         }

common/src/main/java/net/william278/husksync/api/HuskSyncAPI.java

@@ -511,17 +511,19 @@ public class HuskSyncAPI {
      */
     static final class NotRegisteredException extends IllegalStateException {
 
-        private static final String MESSAGE = """
-                Could not access the HuskSync API as it has not yet been registered. This could be because:
+        private static final String REASONS = """
+                This may be because:
                 1) HuskSync has failed to enable successfully
                 2) Your plugin isn't set to load after HuskSync has
                    (Check if it set as a (soft)depend in plugin.yml or to load: BEFORE in paper-plugin.yml?)
-                3) You are attempting to access HuskSync on plugin construction/before your plugin has enabled.
-                4) You have shaded HuskSync into your plugin jar and need to fix your maven/gradle/build script
-                   to only include HuskSync as a dependency and not as a shaded dependency.""";
+                3) You are attempting to access HuskSync on plugin construction/before your plugin has enabled.""";
+
+        NotRegisteredException(@NotNull String reasons) {
+            super("Could not access the HuskSync API as it has not yet been registered. %s".formatted(REASONS));
+        }
 
         NotRegisteredException() {
-            super(MESSAGE);
+            this(REASONS);
         }
 
     }