chore: add InvalidDifficulty variant

src/burnchains/bitcoin/mod.rs

@@ -79,6 +79,8 @@ pub enum Error {
     MissingHeader,
     /// Invalid target
     InvalidPoW,
+    /// Bad difficulty
+    InvalidDifficulty,
     /// Wrong number of bytes for constructing an address
     InvalidByteSequence,
     /// Configuration error
@@ -107,6 +109,7 @@ impl fmt::Display for Error {
             Error::NoncontiguousHeader => write!(f, "Non-contiguous header"),
             Error::MissingHeader => write!(f, "Missing header"),
             Error::InvalidPoW => write!(f, "Invalid proof of work"),
+            Error::InvalidDifficulty => write!(f, "Chain difficulty cannot decrease"),
             Error::InvalidByteSequence => write!(f, "Invalid sequence of bytes"),
             Error::ConfigError(ref e_str) => fmt::Display::fmt(e_str, f),
             Error::BlockchainHeight => write!(f, "Value is beyond the end of the blockchain"),
@@ -133,6 +136,7 @@ impl error::Error for Error {
             Error::NoncontiguousHeader => None,
             Error::MissingHeader => None,
             Error::InvalidPoW => None,
+            Error::InvalidDifficulty => None,
             Error::InvalidByteSequence => None,
             Error::ConfigError(ref _e_str) => None,
             Error::BlockchainHeight => None,