Uncapitalize error message in pkg/kubectl

Following the Go code review comments (https://code.google.com/p/go-wiki/wiki/CodeReviewComments#Error_Strings)

```
Error strings should not be capitalized (unless beginning with proper nouns or acronyms) or end with 
punctuation, since they are usually printed following other context. That is, use fmt.Errorf("something 
bad") not fmt.Errorf("Something bad"), so that log.Print("Reading %s: %v", filename, err) formats without a 
spurious capital letter mid-message. This does not apply to logging, which is implicitly line-oriented and 
not combined inside other messages.
```