expression: add "illegal and/or" for thresholds; drop errstr

This gets rid of the horrible `errstr` function :).

Author: apoelstra

src/expression/error.rs

@@ -212,6 +212,10 @@ pub enum ParseThresholdError {
     NoChildren,
     /// The threshold value appeared to be a sub-expression rather than a number.
     KNotTerminal,
+    /// A 1-of-n threshold was used in a context it was not allowed.
+    IllegalOr,
+    /// A n-of-n threshold was used in a context it was not allowed.
+    IllegalAnd,
     /// Failed to parse the threshold value.
     ParseK(ParseNumError),
     /// Threshold parameters were invalid.
@@ -225,6 +229,12 @@ impl fmt::Display for ParseThresholdError {
         match *self {
             NoChildren => f.write_str("expected threshold, found terminal"),
             KNotTerminal => f.write_str("expected positive integer, found expression"),
+            IllegalOr => f.write_str(
+                "1-of-n thresholds not allowed here; please use an 'or' fragment instead",
+            ),
+            IllegalAnd => f.write_str(
+                "n-of-n thresholds not allowed here; please use an 'and' fragment instead",
+            ),
             ParseK(ref x) => write!(f, "failed to parse threshold value: {}", x),
             Threshold(ref e) => e.fmt(f),
         }
@@ -237,8 +247,7 @@ impl std::error::Error for ParseThresholdError {
         use ParseThresholdError::*;
 
         match *self {
-            NoChildren => None,
-            KNotTerminal => None,
+            NoChildren | KNotTerminal | IllegalOr | IllegalAnd => None,
             ParseK(ref e) => Some(e),
             Threshold(ref e) => Some(e),
         }

src/lib.rs

@@ -628,8 +628,6 @@ impl From<crate::policy::compiler::CompilerError> for Error {
     fn from(e: crate::policy::compiler::CompilerError) -> Error { Error::CompilerError(e) }
 }
 
-fn errstr(s: &str) -> Error { Error::Unexpected(s.to_owned()) }
-
 /// The size of an encoding of a number in Script
 pub fn script_num_size(n: usize) -> usize {
     match n {

src/policy/semantic.rs

@@ -14,8 +14,8 @@ use crate::iter::{Tree, TreeLike};
 use crate::prelude::*;
 use crate::sync::Arc;
 use crate::{
-    errstr, expression, AbsLockTime, Error, ForEachKey, FromStrKey, MiniscriptKey, RelLockTime,
-    Threshold, Translator,
+    expression, AbsLockTime, Error, ForEachKey, FromStrKey, MiniscriptKey, RelLockTime, Threshold,
+    Translator,
 };
 
 /// Abstract policy which corresponds to the semantics of a miniscript and
@@ -346,10 +346,11 @@ impl<Pk: FromStrKey> expression::FromTree for Policy<Pk> {
                 let thresh = top.to_null_threshold().map_err(Error::ParseThreshold)?;
 
                 // thresh(1) and thresh(n) are disallowed in semantic policies
-                if thresh.is_or() || thresh.is_and() {
-                    return Err(errstr(
-                        "Semantic Policy thresh cannot have k = 1 or k = n, use `and`/`or` instead",
-                    ));
+                if thresh.is_or() {
+                    return Err(Error::ParseThreshold(crate::ParseThresholdError::IllegalOr));
+                }
+                if thresh.is_and() {
+                    return Err(Error::ParseThreshold(crate::ParseThresholdError::IllegalAnd));
                 }
 
                 thresh