Use defensive documentation

Only commit in the docs and error messages to what we _really_ know.

In an attempt to reduce the likelyhood of the code going stale only
commit to what is guaranteed - that we have an error from a module.

This does arguably reduce the amount of context around the error.

bitcoin/src/crypto/ecdsa.rs

@@ -192,7 +192,7 @@ pub enum Error {
     SighashType(NonStandardSighashTypeError),
     /// Empty Signature
     EmptySignature,
-    /// secp256k1-related error
+    /// A secp256k1 error
     Secp256k1(secp256k1::Error),
 }
 
@@ -204,7 +204,7 @@ impl fmt::Display for Error {
             Hex(ref e) => write_err!(f, "signature hex decoding error"; e),
             SighashType(ref e) => write_err!(f, "non-standard signature hash type"; e),
             EmptySignature => write!(f, "empty ECDSA signature"),
-            Secp256k1(ref e) => write_err!(f, "invalid ECDSA signature"; e),
+            Secp256k1(ref e) => write_err!(f, "secp256k1"; e),
         }
     }
 }

bitcoin/src/crypto/key.rs

@@ -679,9 +679,9 @@ impl From<TweakedKeyPair> for TweakedPublicKey {
 #[derive(Debug, Clone, PartialEq, Eq)]
 #[non_exhaustive]
 pub enum Error {
-    /// Base58 encoding error
+    /// A base58 error.
     Base58(base58::Error),
-    /// secp256k1-related error
+    /// A secp256k1 error.
     Secp256k1(secp256k1::Error),
     /// Invalid key prefix error
     InvalidKeyPrefix(u8),
@@ -696,10 +696,10 @@ impl fmt::Display for Error {
         use Error::*;
 
         match *self {
-            Base58(ref e) => write_err!(f, "key base58 error"; e),
-            Secp256k1(ref e) => write_err!(f, "key secp256k1 error"; e),
+            Base58(ref e) => write_err!(f, "base58"; e),
+            Secp256k1(ref e) => write_err!(f, "secp256k1"; e),
             InvalidKeyPrefix(ref b) => write!(f, "key prefix invalid: {}", b),
-            Hex(ref e) => write_err!(f, "key hex decoding error"; e),
+            Hex(ref e) => write_err!(f, "hex"; e),
             InvalidHexLength(got) =>
                 write!(f, "pubkey hex should be 66 or 130 digits long, got: {}", got),
         }

bitcoin/src/crypto/taproot.rs

@@ -64,7 +64,7 @@ impl Signature {
 pub enum SigFromSliceError {
     /// Invalid signature hash type.
     SighashType(InvalidSighashTypeError),
-    /// Signature has valid size but does not parse correctly
+    /// A secp256k1 error.
     Secp256k1(secp256k1::Error),
     /// Invalid taproot signature size
     InvalidSignatureSize(usize),
@@ -75,9 +75,8 @@ impl fmt::Display for SigFromSliceError {
         use SigFromSliceError::*;
 
         match *self {
-            SighashType(ref e) => write_err!(f, "invalid signature hash type"; e),
-            Secp256k1(ref e) =>
-                write_err!(f, "taproot signature has correct len but is malformed"; e),
+            SighashType(ref e) => write_err!(f, "sighash"; e),
+            Secp256k1(ref e) => write_err!(f, "secp256k1"; e),
             InvalidSignatureSize(sz) => write!(f, "invalid taproot signature size: {}", sz),
         }
     }