Makes the 'Failed to resolve' error message clearer

Signed-off-by: David Gilligan-Cook <dcook@imageworks.com>

crates/spk-solve/crates/graph/src/error.rs

@@ -103,6 +103,9 @@ impl FormatError for Error {
         match self {
             Error::FailedToResolve(_graph) => {
                 // TODO: provide a summary based on the graph
+                msg.push_str(
+                    ": there is no solution for these requests using the available packages",
+                );
             }
             Error::SolverError(reason) => {
                 msg.push_str("\n * ");

crates/spk-solve/crates/graph/src/graph.rs

@@ -475,7 +475,7 @@ impl<'state, 'cmpt> DecisionBuilder<'state, 'cmpt> {
 }
 
 #[derive(Clone, Debug, Diagnostic, Error)]
-#[error("Failed to resolve")]
+#[error("Failed to resolve: there is no solution for these requests using the available packages")]
 pub struct Graph {
     pub root: Arc<tokio::sync::RwLock<Arc<Node>>>,
     pub nodes: HashMap<u64, Arc<tokio::sync::RwLock<Arc<Node>>>>,

crates/spk-solve/crates/validation/src/error.rs

@@ -41,6 +41,9 @@ impl FormatError for Error {
         match self {
             Error::FailedToResolve(_graph) => {
                 // TODO: provide a summary based on the graph
+                msg.push_str(
+                    ": there is no solution for these requests using the available packages",
+                );
             }
             Error::SolverError(reason) => {
                 msg.push_str("\n * ");

crates/spk-solve/src/solver_test.rs

@@ -887,7 +887,7 @@ async fn test_solver_build_from_source_unsolvable(mut solver: Solver) {
         let msg = strip_ansi_escapes::strip(msg);
         let msg = String::from_utf8_lossy(&msg);
         msg.ends_with(
-            "TRY my-tool/1.2.0/src - cannot resolve build env for source build: Failed to resolve",
+            "TRY my-tool/1.2.0/src - cannot resolve build env for source build: Failed to resolve: there is no solution for these requests using the available packages",
         )
     });
     assert!(