[libcalamaresui] Display a reduced amount of details

Cut the error message from down to a maximum of 8 lines
so that the messagebox does not hopelessly overflow.

src/libcalamaresui/ViewManager.cpp

@@ -136,6 +136,40 @@ ViewManager::insertViewStep( int before, ViewStep* step )
     emit endInsertRows();
 }
 
+/** Makes a long details message suitable for display
+ *
+ * @returns a (possibly shortened) version of @p text that
+ *      will fit sensibly in a messagebox.
+ */
+static QString
+simplifyText( const QString& text )
+{
+    constexpr const int maxLines = 8;
+    constexpr const int maxChars = 640;
+    QString shorter = text.simplified();
+    if ( shorter.count( '\n' ) >= maxLines )
+    {
+        int from = -1;
+        for ( int i = 0; i < maxLines; ++i )
+        {
+            from = shorter.indexOf( '\n', from + 1 );
+            if ( from < 0 )
+            {
+                // That's odd, we counted at least maxLines newlines before
+                break;
+            }
+        }
+        if ( from > 0 )
+        {
+            shorter.truncate( from );
+        }
+    }
+    if ( shorter.length() > maxChars )
+    {
+        shorter.truncate( maxChars );
+    }
+    return shorter;
+}
 
 void
 ViewManager::onInstallationFailed( const QString& message, const QString& details )
@@ -152,7 +186,7 @@ ViewManager::onInstallationFailed( const QString& message, const QString& detail
     QString text = "<p>" + message + "</p>";
     if ( !details.isEmpty() )
     {
-        text += "<p>" + details + "</p>";
+        text += "<p>" + simplifyText( details ) + "</p>";
     }
     if ( shouldOfferWebPaste )
     {