cli: improve argument validation output

Improve the output for these validation errors:

- Removes the short command description from the output. This information
  does not provide much useful help, and distracts from the error message.
- Reduces punctuation, and
- Prefixes the error message with the binary / root-command name
  (usually `docker:`) to be consistent with other similar errors.
- Adds an empty line between the error-message and the "call to action"
  (`Run 'docker volume --help'...` in the example below). This helps
  separating the error message and "usage" from the call-to-action.

Before this patch:

    $ docker volume ls one two three
    "docker volume ls" accepts no arguments.
    See 'docker volume ls --help'.

    Usage:  docker volume ls [OPTIONS]

    List volumes

    $ docker volume create one two three
    "docker volume create" requires at most 1 argument.
    See 'docker volume create --help'.

    Usage:  docker volume create [OPTIONS] [VOLUME]

    Create a volume

With this patch:

    $ docker volume ls one two three
    docker: 'docker volume ls' accepts no arguments

    Usage:  docker volume ls [OPTIONS]

    Run 'docker volume ls --help' for more information

    $ docker voludocker volume create one two three
    docker: 'docker volume create' requires at most 1 argument

    Usage:  docker volume create [OPTIONS] [VOLUME]

    SRun 'docker volume create --help' for more information

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

cli/command/trust/key_generate_test.go

@@ -26,12 +26,12 @@ func TestTrustKeyGenerateErrors(t *testing.T) {
 	}{
 		{
 			name:          "not-enough-args",
-			expectedError: "requires exactly 1 argument",
+			expectedError: "requires 1 argument",
 		},
 		{
 			name:          "too-many-args",
 			args:          []string{"key-1", "key-2"},
-			expectedError: "requires exactly 1 argument",
+			expectedError: "requires 1 argument",
 		},
 	}
 

cli/command/trust/key_load_test.go

@@ -34,13 +34,14 @@ func TestTrustKeyLoadErrors(t *testing.T) {
 	}{
 		{
 			name:           "not-enough-args",
-			expectedError:  "exactly 1 argument",
+			expectedError:  "1 argument",
+			args:           []string{},
 			expectedOutput: "",
 		},
 		{
 			name:           "too-many-args",
 			args:           []string{"iamnotakey", "alsonotakey"},
-			expectedError:  "exactly 1 argument",
+			expectedError:  "1 argument",
 			expectedOutput: "",
 		},
 		{

cli/command/trust/revoke_test.go

@@ -24,12 +24,12 @@ func TestTrustRevokeCommandErrors(t *testing.T) {
 	}{
 		{
 			name:          "not-enough-args",
-			expectedError: "requires exactly 1 argument",
+			expectedError: "requires 1 argument",
 		},
 		{
 			name:          "too-many-args",
 			args:          []string{"remote1", "remote2"},
-			expectedError: "requires exactly 1 argument",
+			expectedError: "requires 1 argument",
 		},
 		{
 			name:          "sha-reference",

cli/command/trust/sign_test.go

@@ -32,12 +32,12 @@ func TestTrustSignCommandErrors(t *testing.T) {
 	}{
 		{
 			name:          "not-enough-args",
-			expectedError: "requires exactly 1 argument",
+			expectedError: "requires 1 argument",
 		},
 		{
 			name:          "too-many-args",
 			args:          []string{"image", "tag"},
-			expectedError: "requires exactly 1 argument",
+			expectedError: "requires 1 argument",
 		},
 		{
 			name:          "sha-reference",