`asdf install` should exit with exit code of 0 when a tool version is set to uninstallable system version

[Stratus3D]

asdf install still returns a non-zero exit code with postgres system in my ~/.tool-versions. I'd think it should only exit with a non-zero exit code when trying to install a system version directly, e.g.: asdf install postgres system as in this test case:

asdf/test/install_command.bats

Lines 220 to 225 in 5646b38

	@test "install_command doesn't install system version" {
	run asdf install dummy system
	[ "$status" -eq 1 ]
	[ "$output" = "error installing version: uninstallable version system of dummy" ]
	[ ! -f "$ASDF_DIR/installs/dummy/system/version" ]
	}

I think asdf install should skip over them and return success.

Currently:

➜ asdf install
[...]
skipping uninstallable version system of postgres
version [...] of ruby is already installed
[...]

➜ echo $?                     
1

➜ asdf install postgres system
error installing version: uninstallable version system of postgres

➜ echo $?                     
1

Originally posted by @ZimbiX in #1930