LibCore: Use ErrorOr<T> for Core::File::open()

Userland/Applications/Spreadsheet/ExportDialog.cpp

@@ -22,6 +22,7 @@
 #include <LibGUI/TextBox.h>
 #include <LibGUI/Wizards/WizardDialog.h>
 #include <LibGUI/Wizards/WizardPage.h>
+#include <string.h>
 #include <unistd.h>
 
 // This is defined in ImportDialog.cpp, we can't include it twice, since the generated symbol is exported.

Userland/Applications/Spreadsheet/Readers/Test/TestXSV.cpp

@@ -10,6 +10,7 @@
 #include "../XSV.h"
 #include <AK/ByteBuffer.h>
 #include <LibCore/File.h>
+#include <string.h>
 
 TEST_CASE(should_parse_valid_data)
 {

Userland/Applications/Spreadsheet/Workbook.cpp

@@ -51,7 +51,7 @@ Result<bool, String> Workbook::load(const StringView& filename)
         sb.append("Failed to open ");
         sb.append(filename);
         sb.append(" for reading. Error: ");
-        sb.append(file_or_error.error().string());
+        sb.appendff("{}", file_or_error.error());
 
         return sb.to_string();
     }