style: Fix staticcheck (static analysis) errors for this library (#283)

* style: Fix staticcheck errors for "error strings should not be capitalized (ST1005)"

staticcheck is a static analysis tool for go.
It reports several "error strings should not be capitalized (ST1005)" messages.
Here, we fix it to be more compliant with the go coding styleguide.

Related: #280

* style: Fix staticcheck errors for "printf-style function with dynamic format ... (SA1006)"

staticcheck is a static analysis tool for go.
It reports several "printf-style function with dynamic format string and no further arguments should use print-style function instead (SA1006)" messages.
Here, we fix it to be more compliant with the go coding styleguide.

Related: #280

* style: Fix staticcheck errors for "type X is unused (U1000)"

staticcheck is a static analysis tool for go.
It reports several "type X is unused (U1000)" messages.
Here, we fix it to be more compliant with the go coding styleguide.

Related: #280

* style: Fix staticcheck errors for "should use X instead (S1003 & SA6005)"

staticcheck is a static analysis tool for go.
It reports several

- should use !bytes.Contains(b, []byte(`"password":"bar"`)) instead (S1003)
- should use strings.EqualFold instead (SA6005)

messages.
Here, we fix it to be more compliant with the go coding styleguide.

Related: #280

* style: Fix staticcheck errors for "unnecessary use of fmt.Sprintf (S1039)"

staticcheck is a static analysis tool for go.
It report several "unnecessary use of fmt.Sprintf (S1039)" messages.
Here, we fix it to be more compliant with the go coding styleguide.

Related: #280

* style: Fix staticcheck errors for "this value of X is never used (SA4006)"

staticcheck is a static analysis tool for go.
It report several "this value of X is never used (SA4006)" messages.
Here, we fix it to be more compliant with the go coding styleguide.

Related: #280

* style: Fix staticcheck errors for "redundant return statement (S1023)"

staticcheck is a static analysis tool for go.
It report several "redundant return statement (S1023)" messages.
Here, we fix it to be more compliant with the go coding styleguide.

Related: #280

* style: Fix staticcheck errors for "possible nil pointer dereference (SA5011)"

staticcheck is a static analysis tool for go.
It report several

    file.go:Line:character: possible nil pointer dereference (SA5011)
        file.go:Line:character: this check suggests that the pointer can be nil

messages.
Here, we fix it to be more compliant with the go coding styleguide.

Related: #280

* style: Fix staticcheck errors for "this value of X is never used (SA4006)"

staticcheck is a static analysis tool for go.
It report several "this value of X is never used (SA4006)" messages.
Here, we fix it to be more compliant with the go coding styleguide.

Related: #280

role.go

@@ -69,7 +69,7 @@ func (s *RoleService) Get(roleID int) (*Role, *Response, error) {
 		return nil, resp, jerr
 	}
 	if role.Self == "" {
-		return nil, resp, fmt.Errorf("No role with ID %d found", roleID)
+		return nil, resp, fmt.Errorf("no role with ID %d found", roleID)
 	}
 
 	return role, resp, err