Proposal of fixing error string

[yyh-gl]

I think that error string should not be capitalized or end with punctuation. This idea is based on golint (ref)

Therefore, I propose the fix below. https://github.com/dgrijalva/jwt-go/pull/391

Of course, I understand that this change is difficult to merge because it is not backward compatible.

However, I believe this fix is ​​important for gopher because it's common to comply with golint in golang.