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

README.md

@@ -119,10 +119,12 @@ order; the first one found wins:
 4. A prompt on the terminal with echo turned off.
 
 If none of these is available and standard input is not a terminal, the tool
-refuses and exits with status 1. A mnemonic that fails the BIP-39 checksum is
-refused with a message saying so. Keys are derived from the mnemonic's words
-joined by single spaces, whatever whitespace is around or between them, so one
-word per line, tabs or extra spaces give the same keys.
+refuses and exits with status 1. A mnemonic that is empty, does not have 12, 15,
+18, 21 or 24 words, has a word that is not on the BIP-39 word list, or fails the
+BIP-39 checksum is refused with a message saying which. A word that is not on
+the list is named by its position, never printed. Keys are derived from the
+mnemonic's words joined by single spaces, whatever whitespace is around or
+between them, so one word per line, tabs or extra spaces give the same keys.
 
 `KEYFUNC_MNEMONIC` and `KEYFUNC_MNEMONIC_COMMAND` are removed from the
 environment before the mnemonic command, the system `ssh` (`keyfunc ssh to`) and