Fix empty error being reported for child process

Sometimes stderr gets empty strings. Now we will wait for the program to exit and display all stderr if it's not empty

src/functions-cli.ts

@@ -24,11 +24,22 @@ export class FunctionsCli {
                 cwd: workingDirectory
             };
             const childProc = cp.spawn('func', args, options);
+            let stderr: string = '';
             childProc.stdout.on('data', (data) => outputChannel.append(data.toString()));
-            childProc.stderr.on('data', (data) => reject(new Error(data.toString())));
+            childProc.stderr.on('data', (data) => stderr = stderr.concat(data.toString()));
             childProc.on('error', error => reject(error));
             childProc.on('close', code => {
-                code === 0 ? resolve() : reject(new Error(`Command failed with exit code '${code}'.`))
+                const errorMessage = stderr.trim();
+                if (errorMessage) {
+                    // TODO: 'func' commands always seem to return exit code 0. For now,
+                    // we will use stderr to detect if an error occurs (even though sterr
+                    // doesn't necessarily mean there's an error)
+                    reject(errorMessage);
+                } else if (code !== 0) {
+                    reject(new Error(`Command failed with exit code '${code}'.`));
+                } else {
+                    resolve();
+                }
             });
         });
     }