Say why a mnemonic is refused (closes #76)

Every way a mnemonic could be invalid was reported as a checksum
failure, including an empty one. It is now refused as empty, as having
a word count other than 12, 15, 18, 21 or 24, as having a word that is
not on the BIP-39 word list, or as failing the checksum. A word not on
the list is named by its position, never by the word itself.

Model: opus-5-5

internal/mnemonic/mnemonic_test.go

@@ -105,6 +105,38 @@ func TestABadChecksumIsRefused(t *testing.T) {
 	require.ErrorIs(t, err, mnemonic.ErrChecksum)
 }
 
+func TestACommandThatPrintsNothingGivesAnEmptyMnemonic(t *testing.T) {
+	t.Setenv(mnemonic.CommandVariable, "")
+	t.Setenv(mnemonic.Variable, "")
+
+	_, err := mnemonic.Read(t.Context(), "true")
+	require.ErrorIs(t, err, mnemonic.ErrEmpty)
+	require.NotContains(t, err.Error(), "checksum")
+}
+
+func TestAWrongWordCountIsRefused(t *testing.T) {
+	t.Setenv(mnemonic.CommandVariable, "")
+	t.Setenv(mnemonic.Variable, strings.Repeat("abandon ", 3))
+
+	_, err := mnemonic.Read(t.Context(), "")
+	require.ErrorIs(t, err, mnemonic.ErrWordCount)
+	require.ErrorContains(t, err, "not 3")
+	require.NotContains(t, err.Error(), "checksum")
+}
+
+func TestAWordNotOnTheListIsNamedByPositionOnly(t *testing.T) {
+	const typo = "abuot"
+
+	t.Setenv(mnemonic.CommandVariable, "")
+	t.Setenv(mnemonic.Variable, strings.Repeat("abandon ", 11)+typo)
+
+	_, err := mnemonic.Read(t.Context(), "")
+	require.ErrorIs(t, err, mnemonic.ErrUnknownWord)
+	require.ErrorContains(t, err, "word 12 ")
+	require.NotContains(t, err.Error(), typo)
+	require.NotContains(t, err.Error(), "checksum")
+}
+
 func TestNothingToReadAndNobodyToAsk(t *testing.T) {
 	t.Setenv(mnemonic.CommandVariable, "")
 	t.Setenv(mnemonic.Variable, "")