Improves error handling in the case of missing type

sqlx-core/src/error.rs

@@ -57,6 +57,10 @@ pub enum Error {
     #[error("no rows returned by a query that expected to return at least one row")]
     RowNotFound,
 
+    /// Type in query doesn't exist. Likely due to typo or missing user type.
+    #[error("type named {type_name} not found")]
+    TypeNotFound { type_name: String },
+
     /// Column index was out of bounds.
     #[error("column index out of bounds: the len is {len}, but the index is {index}")]
     ColumnIndexOutOfBounds { index: usize, len: usize },

sqlx-core/src/postgres/connection/describe.rs

@@ -235,11 +235,13 @@ SELECT oid FROM pg_catalog.pg_type WHERE typname ILIKE $1
                 ",
         )
         .bind(name)
-        .fetch_one(&mut *self)
-        .await?;
+        .fetch_optional(&mut *self)
+        .await?
+        .ok_or_else(|| Error::TypeNotFound {
+            type_name: String::from(name),
+        })?;
 
         self.cache_type_oid.insert(name.to_string().into(), oid);
-
         Ok(oid)
     }