kernel: SanityChecks: Return an error struct

This reduces libbitcoinkernel's coupling with ui_interface and
translation.

src/kernel/checks.cpp

@@ -5,29 +5,26 @@
 #include <kernel/checks.h>
 
 #include <key.h>
-#include <node/ui_interface.h>
 #include <random.h>
 #include <util/time.h>
-#include <util/translation.h>
-
-#include <memory>
 
 namespace kernel {
 
-bool SanityChecks(const Context&) {
+std::optional<SanityCheckError> SanityChecks(const Context&)
+{
     if (!ECC_InitSanityCheck()) {
-        return InitError(Untranslated("Elliptic curve cryptography sanity check failure. Aborting."));
+        return SanityCheckError::ERROR_ECC;
     }
 
     if (!Random_SanityCheck()) {
-        return InitError(Untranslated("OS cryptographic RNG sanity check failure. Aborting."));
+        return SanityCheckError::ERROR_RANDOM;
     }
 
     if (!ChronoSanityCheck()) {
-        return InitError(Untranslated("Clock epoch mismatch. Aborting."));
+        return SanityCheckError::ERROR_CHRONO;
     }
 
-    return true;
+    return std::nullopt;
 }
 
 }