pep440: some minor refactoring, mostly around error types (#780)

This PR does a bit of refactoring to the pep440 crate, and in
particular around the erorr types. This PR is meant to be a precursor
to another PR that does some surgery (both in parsing and in `Version`
representation) that benefits somewhat from this refactoring.

As usual, please review commit-by-commit.

crates/pep440-rs/src/lib.rs

@@ -44,7 +44,9 @@
 
 pub use {
     version::{LocalSegment, Operator, PreRelease, Version},
-    version_specifier::{parse_version_specifiers, VersionSpecifier, VersionSpecifiers},
+    version_specifier::{
+        parse_version_specifiers, VersionSpecifier, VersionSpecifiers, VersionSpecifiersParseError,
+    },
 };
 
 #[cfg(feature = "pyo3")]
@@ -55,30 +57,6 @@ pub use version::PyVersion;
 mod version;
 mod version_specifier;
 
-/// Error with span information (unicode width) inside the parsed line
-#[derive(Debug, Eq, PartialEq, Clone)]
-pub struct Pep440Error {
-    /// The actual error message
-    pub message: String,
-    /// The string that failed to parse
-    pub line: String,
-    /// First character for underlining (unicode width)
-    pub start: usize,
-    /// Number of characters to underline (unicode width)
-    pub width: usize,
-}
-
-impl std::fmt::Display for Pep440Error {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        writeln!(f, "Failed to parse version:")?;
-        writeln!(f, "{}", self.line)?;
-        writeln!(f, "{}{}", " ".repeat(self.start), "^".repeat(self.width))?;
-        Ok(())
-    }
-}
-
-impl std::error::Error for Pep440Error {}
-
 /// Python bindings shipped as `pep440_rs`
 #[cfg(feature = "pyo3")]
 #[pymodule]