Fix format specifier (add %s for actual error msgs) in errors

Hi

This PR fixes errors messages formatting.

Before:

```
mzh@Mzh-laptop:~/Development/go/src/github.com/cloudfoundry-community/cfplayground$ go run main.go 
Starting web ui on http://localhost:8080
Fatal Error
Error logging in as admin: %!(EXTRA *exec.ExitError=exit status 1)
exit status 1
```

After:

```
mzh@Mzh-laptop:~/Development/go/src/github.com/cloudfoundry-community/cfplayground$ go run main.go 
Starting web ui on http://localhost:8080
Fatal Error
Error logging in as admin: exit status 1
exit status 1
```