Prior to the fix, the commands didn't return the proper exit code in the case of an error and didn't correctly handle the error exit code of the underlying `env` command.

**Fish:**
```
$ phpbrew use blah
PHP version blah is not installed.

$ echo $status
0

$ phpbrew switch blah
Invalid: command not found
~/.phpbrew/init (line 2):
Invalid argument blah
^
from sourcing file ~/.phpbrew/init
	called on line 794 of file ~/.phpbrew/phpbrew.fish

in function “__phpbrew_update_config”
	called on line 807 of file ~/.phpbrew/phpbrew.fish
	with parameter list “blah”

in function “__phpbrew_reinit”
	called on line 397 of file ~/.phpbrew/phpbrew.fish
	with parameter list “blah”

in function “phpbrew”
	called on standard input
	with parameter list “switch blah”

$ echo $status
0

$ cat ~/.phpbrew/init
Invalid argument blah
```

**Bash:**
```
$ phpbrew use blah
PHP version blah is not installed.

$ echo $?
0

$ phpbrew switch blah
Invalid: command not found

$ echo $?
0

$ cat ~/.phpbrew/init
Invalid argument blah
```