opts: remove ErrBadKey as it's not used as a sentinel error

This error was originally introduced `ErrBadEnvVariable` in [moby/moby@500c8ba],
but merely for convenience, and not used as a sentinel error. After the code
was moved from the daemon to the cli repository, it was renamed to be more
generic `ErrBadKey` in commit 2b17f4c8a8.

A search on GitHub shows that there's no consumers using this error as
sentinel error, and it's not used in our own code as such, so it should
be safe to remove this error.

This patch removes the `ErrBadKey` error-type; it also removes the prefix
(`poorly formatted environment:`) to make the error more generic, because
the same function was used both for env-files and label-files.

[moby/moby@500c8ba]: 500c8ba4b6

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

opts/file.go

@@ -12,15 +12,6 @@ import (
 
 const whiteSpaces = " \t"
 
-// ErrBadKey typed error for bad environment variable
-type ErrBadKey struct {
-	msg string
-}
-
-func (e ErrBadKey) Error() string {
-	return "poorly formatted environment: " + e.msg
-}
-
 func parseKeyValueFile(filename string, emptyFn func(string) (string, bool)) ([]string, error) {
 	fh, err := os.Open(filename)
 	if err != nil {
@@ -51,10 +42,10 @@ func parseKeyValueFile(filename string, emptyFn func(string) (string, bool)) ([]
 			// trim the front of a variable, but nothing else
 			variable = strings.TrimLeft(variable, whiteSpaces)
 			if strings.ContainsAny(variable, whiteSpaces) {
-				return []string{}, ErrBadKey{fmt.Sprintf("variable '%s' contains whitespaces", variable)}
+				return []string{}, fmt.Errorf("variable '%s' contains whitespaces", variable)
 			}
 			if len(variable) == 0 {
-				return []string{}, ErrBadKey{fmt.Sprintf("no variable name on line '%s'", line)}
+				return []string{}, fmt.Errorf("no variable name on line '%s'", line)
 			}
 
 			if hasValue {