Issue #1397: File contents are now kept if load fails, add "no pages" loading error

The "Rephrase error message (thanks to Technius!)" commit on the issue_1397_archive branch is included in this PR.
Because that change is minor, I squashed everything into 1 commit.

src/control/Control.cpp

@@ -2316,8 +2316,6 @@ bool Control::openFile(Path filename, int scrollToPage, bool forceOpen)
 		}
 	}
 
-	this->closeDocument();
-
 	// Read template file
 	if (filename.hasExtension(".xopt"))
 	{
@@ -2381,6 +2379,8 @@ bool Control::openFile(Path filename, int scrollToPage, bool forceOpen)
 	}
 	else
 	{
+		this->closeDocument();
+
 		this->doc->lock();
 		this->doc->clearDocument();
 		*this->doc = *loadedDocument;

src/control/xojfile/LoadHandler.cpp

@@ -298,6 +298,11 @@ bool LoadHandler::parseXml()
 		lastError = _("Document is not complete (maybe the end is cut off?)");
 		return false;
 	}
+	else if (this->pos == PASER_POS_FINISHED && this->doc.getPageCount() == 0)
+	{
+		lastError = _("Document is corrupted (no pages found in file)");
+		return false;
+	}
 
 	doc.setCreateBackupOnSave(this->fileVersion >= 3);