Doesn't warn when trying to install to msdos disk in EFI mode

**Describe the bug**
When I tried to install elementary OS on a separate hard disk I used custom partitioning on an external hard drive that has an `msdos` partition table. I selected my root and my boot partition, and proceeded to install elementary OS. The bar was stuck at 0% for quite a while, so I opened the terminal view to see what's happening. Apparently, some component of the installer refused to install the OS on UEFI mode because the disk didn't have a `gpt` partition table. At that point though, there was no way to close the installer, since it didn't detect the error itself, and there was also no cancel button. I had to go to gparted and redo everything after I changed the partition table.

**To Reproduce**
Steps to reproduce the behavior:
1. Open the installer, choose language/locale/etc.
2. Choose custom install
3. Choose the boot and root partitions in a disk with `msdos` partition table
4. Start the installation
5. Installer stuck at 0%
6. Open the terminal view
7. see the error messages

**Expected behavior**
I believe the most proper way to prevent this would be to make it a requirement in the custom installation page. Show that the device is connected, but gray it out to show that it's an unavailable installation destination because of its partition table. A UEFI installation requires GPT after all.

It's also very important to catch errors during installation and show them to the user when they happen.

**Desktop (please complete the following information):**
- OS: elementary OS 6 beta (on external hard drive) & Debian Bullseye
- Browser: Firefox & Firefox ESR

**Smartphone (please complete the following information):**
- Device: Xiaomi Redmi Note 5 Pro
- OS: MIUI Global 12.0.2, based on Android version 9
- Browser: Vivaldi