Update gemini error msg (#3847)

Co-authored-by: Douwe Osinga <douwe@squareup.com>

crates/goose/src/providers/gemini_cli.rs

@@ -176,9 +176,13 @@ impl GeminiCliProvider {
 
         cmd.stdout(Stdio::piped()).stderr(Stdio::piped());
 
-        let mut child = cmd
+        let mut child = cmd.spawn().map_err(|e| {
-            .spawn()
+            ProviderError::RequestFailed(format!(
-            .map_err(|e| ProviderError::RequestFailed(format!("Failed to spawn command: {}", e)))?;
+                "Failed to spawn Gemini CLI command '{}': {}. \
+                Make sure the Gemini CLI is installed and in your PATH.",
+                self.command, e
+            ))
+        })?;
 
         let stdout = child
             .stdout