Cleanup LedgerEntryType & TxType:

This commit removes the `ltINVALID` pseudo-type identifier from
`LedgerEntryType` and the `ttINVALID` pseudo-type identifier from
`TxType` and includes several small additional improvements that
help to simplify the code base.

It also improves the documentation `LedgerEntryType` and `TxType`,
which was all over the place, and highlights some important caveats
associated with making changes to the ledger and transaction type
identifiers.

The commit also adds a safety check to the `KnownFormats<>` class,
that will catch the the accidental reuse of format identifiers.
Ideally, this should be done at compile time but C++ does not (yet?)
allow for the sort of introspection that would enable this.

src/ripple/protocol/impl/STLedgerEntry.cpp

@@ -32,22 +32,12 @@ namespace ripple {
 STLedgerEntry::STLedgerEntry(Keylet const& k)
     : STObject(sfLedgerEntry), key_(k.key), type_(k.type)
 {
-    // The on-ledger representation of a key type is a 16-bit unsigned integer
-    // but the LedgerEntryType enum has a larger range (including negative
-    // values), so catch obviously wrong values:
-    constexpr auto const minValidValue =
-        static_cast<LedgerEntryType>(std::numeric_limits<std::uint16_t>::min());
-
-    constexpr auto const maxValidValue =
-        static_cast<LedgerEntryType>(std::numeric_limits<std::uint16_t>::max());
-
-    if (type_ < minValidValue || type_ > maxValidValue)
-        Throw<std::runtime_error>("invalid ledger entry type: out of range");
-
     auto const format = LedgerFormats::getInstance().findByType(type_);
 
     if (format == nullptr)
-        Throw<std::runtime_error>("unknown ledger entry type");
+        Throw<std::runtime_error>(
+            "Attempt to create a SLE of unknown type " +
+            std::to_string(safe_cast<std::uint16_t>(k.type)));
 
     set(format->getSOTemplate());