refactor!: remove Error::UntrustedMessage

Replaced with `InvalidMessage` which is more appropriate.

`UntrustedMessage` was supposed to mean that the message is otherwise valid but we don't trust the key it was signed with. However, we often used this error even in cases where the message was actually invalid, for example when the proof chain was broken. Also, untrusted messages are bounced back to the sender, so receiving them is not treated as an error anyway.

BREAKING CHANGE: this affects the `Error` type which is a part of the public API.

src/error.rs

@@ -33,8 +33,6 @@ pub enum Error {
     InvalidDestination,
     #[error("Content of a received message is inconsistent.")]
     InvalidMessage,
-    #[error("A signed message could not be trusted.")]
-    UntrustedMessage,
     #[error("A signature share is invalid.")]
     InvalidSignatureShare,
     #[error("The secret key share is missing.")]

src/routing/bootstrap.rs

@@ -441,7 +441,7 @@ impl<'a> State<'a> {
             .verify(trusted_key.map(|key| (&prefix, key)))
             .and_then(|status| match (status, trusted_key) {
                 (VerifyStatus::Full, _) | (VerifyStatus::Unknown, None) => Ok(()),
-                (VerifyStatus::Unknown, Some(_)) => Err(Error::UntrustedMessage),
+                (VerifyStatus::Unknown, Some(_)) => Err(Error::InvalidMessage),
             });
 
         match result {

src/section/mod.rs

@@ -45,7 +45,8 @@ impl Section {
     /// (`elders_info`).
     pub fn new(chain: SectionProofChain, elders_info: Proven<EldersInfo>) -> Result<Self, Error> {
         if !chain.has_key(&elders_info.proof.public_key) {
-            return Err(Error::UntrustedMessage);
+            // TODO: consider more specific error here.
+            return Err(Error::InvalidMessage);
         }
 
         Ok(Self {
@@ -86,6 +87,8 @@ impl Section {
         Ok((section, section_key_share))
     }
 
+    /// Try to merge this `Section` with `other`. Returns `InvalidMessage` if `other` is invalid or
+    /// its chain is not compatible with the chain of `self`.
     pub fn merge(&mut self, other: Self) -> Result<()> {
         if !other.chain.self_verify() || !other.elders_info.verify(&other.chain) {
             return Err(Error::InvalidMessage);
@@ -94,7 +97,7 @@ impl Section {
         // TODO: handle forks
         self.chain
             .merge(other.chain)
-            .map_err(|_| Error::UntrustedMessage)?;
+            .map_err(|_| Error::InvalidMessage)?;
 
         match cmp_section_chain_position(
             &self.elders_info.proof,