Missing space on partition with Steam Library leads to wrong error message "need execute permissions"

[ljrk0]

Your system information

• Steam client version (build number or date): Jul 2 2019, at 01:38:08
• Distribution (e.g. Ubuntu): N/A (ArchLinux)
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: Yes

Please describe your issue in as much detail as possible:

The test for exec permissions tries to deploy a script on the directory which fails if no space is left on the device, as can be seen in the log:

Couldn't write /mnt/ssd_data/SteamLibrary/steamapps/.steam_exec_test.sh: No space left on device

However the message box for the user incorrectly tells them to check for execute permissions, presumably because the return value of the whole routine is checked. It would make more sense to directly propagate the actual error.

Steps for reproducing this issue:

1. Mount a device with a Steam Library but no space left (and with exec permissions)
2. Start Steam from the console
3. Try to add the Steam Library and observe the different error messages on console and message box.

[Sokol08]

Confirm for exFAT file system, for ext4,ntfs work correct, for Win10 work is correct on exFat