Add AlertDialog displaying specific error message when categories search API call returns an error

app/src/main/java/fr/free/nrw/commons/upload/categories/CategoriesContract.kt

@@ -17,6 +17,11 @@ interface CategoriesContract {
 
         fun showError(stringResourceId: Int)
 
+        /**
+         * Show a cancelable AlertDialog with a given message.
+         */
+        fun showErrorDialog(message: String)
+
         fun setCategories(categories: List<CategoryItem>?)
 
         fun goToNextScreen()

app/src/main/java/fr/free/nrw/commons/upload/categories/CategoriesPresenter.kt

@@ -12,6 +12,7 @@ import fr.free.nrw.commons.di.CommonsApplicationModule.Companion.IO_THREAD
 import fr.free.nrw.commons.di.CommonsApplicationModule.Companion.MAIN_THREAD
 import fr.free.nrw.commons.repository.UploadRepository
 import fr.free.nrw.commons.upload.depicts.proxy
+import fr.free.nrw.commons.wikidata.mwapi.MwIOException
 import io.reactivex.Observable
 import io.reactivex.Scheduler
 import io.reactivex.android.schedulers.AndroidSchedulers
@@ -75,7 +76,12 @@ class CategoriesPresenter
                         },
                         { t: Throwable? ->
                             view.showProgress(false)
-                            view.showError(R.string.no_categories_found)
+                            view.showError(R.string.error_loading_categories)
+                            val mwException = t as? MwIOException
+                            view.showErrorDialog(
+                                if (mwException == null) ""
+                                else "\n${mwException.error.title} / ${mwException.error.details}"
+                            )
                             Timber.e(t)
                         },
                     ),

app/src/main/java/fr/free/nrw/commons/upload/categories/UploadCategoriesFragment.kt

@@ -10,6 +10,7 @@ import android.view.LayoutInflater
 import android.view.View
 import android.view.ViewGroup
 import android.widget.Toast
+import androidx.appcompat.app.AlertDialog
 import androidx.appcompat.app.AppCompatActivity
 import androidx.recyclerview.widget.LinearLayoutManager
 import com.jakewharton.rxbinding2.view.RxView
@@ -32,7 +33,6 @@ import io.reactivex.Notification
 import io.reactivex.android.schedulers.AndroidSchedulers
 import io.reactivex.disposables.Disposable
 import timber.log.Timber
-import java.util.Objects
 import java.util.concurrent.TimeUnit
 import javax.inject.Inject
 
@@ -199,6 +199,15 @@ class UploadCategoriesFragment : UploadBaseFragment(), CategoriesContract.View {
         binding?.tilContainerSearch?.error = getString(stringResourceId)
     }
 
+    override fun showErrorDialog(message: String) {
+        AlertDialog
+            .Builder(requireContext())
+            .setMessage(getString(R.string.error_loading_categories) + "\n" + message)
+            .setCancelable(false)
+            .setNegativeButton(R.string.ok){_,_ -> }
+            .show()
+    }
+
     override fun setCategories(categories: List<CategoryItem>?) {
         if (adapter == null) {
             Timber.e("Adapter is null in setCategories")