Ensure error messages don't leak private key

Since #8766, invalid base64 is rendered in errors, but we don't actually
want to show this in the case of an invalid private keys.

Co-Authored-By: Eelco Dolstra <edolstra@gmail.com>
(cherry picked from commit 2b6b03d8df)

src/libutil/util.cc

@@ -261,7 +261,7 @@ std::string base64Decode(std::string_view s)
 
         char digit = base64DecodeChars[(unsigned char) c];
         if (digit == npos)
-            throw Error("invalid character in Base64 string: '%c'", c);
+            throw FormatError("invalid character in Base64 string: '%c'", c);
 
         bits += 6;
         d = d << 6 | digit;