move config loading to config module

This separates concerns a bit more. We will probably want to extend the
logic for config loading in the future, and that stuff should all live
in the relevant place. This change points us in the right direction.

src/error.rs

@@ -1,6 +1,6 @@
 //! Error handling facilities
 
-use std::{fmt, iter};
+use std::{fmt, iter, path::PathBuf};
 
 use thiserror::Error;
 
@@ -38,11 +38,8 @@ impl fmt::Display for DisplayWithSources<'_> {
 #[allow(missing_docs)]
 #[derive(Error, Debug)]
 pub(crate) enum Main {
-    #[error("failed to read configuration file")]
-    ConfigRead(#[source] std::io::Error),
-
-    #[error("failed to parse configuration")]
-    ConfigParse(#[from] toml::de::Error),
+    #[error("failed to load configuration")]
+    Config(#[from] Config),
 
     #[error("failed to initialize observability")]
     Observability(#[from] Observability),
@@ -74,3 +71,16 @@ pub(crate) enum Observability {
     #[error("tracing_flame error")]
     TracingFlame(#[from] tracing_flame::Error),
 }
+
+/// Configuration errors
+// Missing docs are allowed here since that kind of information should be
+// encoded in the error messages themselves anyway.
+#[allow(missing_docs)]
+#[derive(Error, Debug)]
+pub(crate) enum Config {
+    #[error("failed to read configuration file {1:?}")]
+    Read(#[source] std::io::Error, PathBuf),
+
+    #[error("failed to parse configuration file {1:?}")]
+    Parse(#[source] toml::de::Error, PathBuf),
+}