add some error handling, mostly to quiet warnings

bffhd/db/mod.rs

@@ -1,5 +1,8 @@
 use thiserror::Error;
 
+// for converting a database error into a failed promise
+use capnp;
+
 mod raw;
 
 use miette::{Diagnostic, Severity};
@@ -79,3 +82,9 @@ impl Diagnostic for Error {
         None
     }
 }
+
+impl From<Error> for capnp::Error {
+    fn from(dberr: Error) -> capnp::Error {
+        capnp::Error::failed(format!("database error: {}", dberr.to_string()))
+    }
+}