HiDPI on external monitor (Dell u2515h) with Macbook Pro

You can try Display Maestro :

Unlock all display resolutions Display Maestro gives you full control over attached displays, allowing the usage of all available resolutions and bit depths. This is done by ignoring the operating system setting of hiding potentially unsafe resolutions.

Restore legacy depth modes It is ideal for running legacy games which require 256-color mode and do not automatically switch the screen depth for you. Mac OS X 10.5 Leopard removed the ability to set 256 color mode manually using the built-in preferences pane, but the feature is now available here in Display Maestro!

Light-weight and background-friendly Display Maestro is a small application which can be configured to launch during login, and sits quietly in the menu bar. Upon exit, it will also automatically revert your display back to the resolution chosen before it was launched.

I had exactly the same problem, with the same monitor, Dell U2713 / U2713H / U2713HM / U2713HMt. I had two of them plugged in to my late-2014 Retina Macbook Pro 15" and only one was experiencing the problem. I tried 100s of different solutions (I can say for sure it's definitely not the cable!) and nothing worked. Just like you, I could get landscape 2560x1440 but not portrait. And I was getting the same error WindowServer: description_for_mode_number: IOFBGetPixelInformation failed (-536870206) when I tried to switch to portrait. That is, until I came across this: https://discussions.apple.com/thread/5485930?start=0&tstart=0. I regenerated the EDID override for the screen using the Ruby script, copied it into the correct location, rebooted and now it works again. I think I caused the problem by playing a game (Trine 2) in vertical (720x1280) resolution - after that, I tried everything to get the screen back to displaying portrait 1440x2560 but to no avail. Until I got the Ruby script from the thread above which fixed it.

Just to note, in case the linked thread disappears... the override file is in /System/Library/Displays/Overrides/ under the folder DisplayVendorID-10ac which is for Dell displays. The file for the U2713-series of monitors (and I think the U2711-series too) is DisplayProductID-4080. The contents of the file should be:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>DisplayProductName</key>
  <string>Dell U2713HMt</string>
  <key>IODisplayEDID</key>
  <data>AP///////wAQrIBATDA1NB4WAQSlPCJ4Iku1p1ZLoyUKUFSlSwCBALMA0QBx
T6lAgYABAQEBVl4AoKCgKVAwIDUAVVAhAAAaAAAA/wBHSzBLRDI3TjQ1MEwK
AAAA/ABERUxMIFUyNzEzSE0KAAAA/QAxVh1xHgEKICAgICAgAA0=
</data>
  <key>DisplayVendorID</key>
  <integer>4268</integer>
  <key>DisplayProductID</key>
  <integer>16512</integer>
</dict>
</plist>