Update crateNameFromCargoToml with better fallback error messages

* If a derivation is created without a name *and* we cannot infer a name
  from a Cargo.toml file, we'll throw a descriptive error message which
  hints towards the remediation
* Otherwise nix can throw a pretty obtuse "derivation has no name" error
  with no error trace

lib/crateNameFromCargoToml.nix

@@ -2,12 +2,24 @@
 
 { src ? null
 , cargoToml ? src + /Cargo.toml
-, cargoTomlContents ? builtins.readFile cargoToml
+, cargoTomlContents ? null
 , ...
 }:
 
+if cargoTomlContents == null && (src == null || !builtins.pathExists cargoToml)
+then
+  throw ''
+    unable to infer crate name and version. Please make sure the src directory
+    contains a valid Cargo.toml file, or consider setting a derivation name explicitly
+  ''
+else
   let
-  toml = fromTOML cargoTomlContents;
+    cargoTomlRealContents =
+      if cargoTomlContents != null
+      then cargoTomlContents
+      else builtins.readFile cargoToml;
+
+    toml = fromTOML cargoTomlRealContents;
     p = toml.package;
   in
   {