[Ruby] Add error output (OpenAPITools#5428)

* Add error output to the log so that we can make sure why the error occurred

* Fix forbidden method invocation using default charsets

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractRubyCodegen.java

@@ -27,7 +27,10 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import java.io.BufferedReader;
 import java.io.File;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 import java.util.Locale;
 
@@ -200,7 +203,13 @@ public void postProcessFile(File file, String fileType) {
                 Process p = Runtime.getRuntime().exec(command);
                 int exitValue = p.waitFor();
                 if (exitValue != 0) {
-                    LOGGER.error("Error running the command ({}). Exit value: {}", command, exitValue);
+                    BufferedReader br = new BufferedReader(new InputStreamReader(p.getErrorStream(), StandardCharsets.UTF_8));
+                    StringBuilder sb = new StringBuilder();
+                    String line;
+                    while ((line = br.readLine()) != null) {
+                        sb.append(line);
+                    }
+                    LOGGER.error("Error running the command ({}). Exit value: {}, Error output: {}", command, exitValue, sb.toString());
                 } else {
                     LOGGER.info("Successfully executed: " + command);
                 }