TurboJPEG: Improve error handling

- Provide a new C API function and TJException method that allows
  calling programs to query the severity of a compression/decompression/
  transform error.
- Provide a new flag that instructs the library to immediately stop
  compressing/decompressing/transforming if a warning is encountered.

Fixes #151

java/org/libjpegturbo/turbojpeg/TJException.java

@@ -1,5 +1,6 @@
 /*
  * Copyright (C)2015 Viktor Szathmáry.  All Rights Reserved.
+ * Copyright (C)2017 D. R. Commander.  All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -46,8 +47,26 @@ public class TJException extends IOException {
     super(message);
   }
 
+  public TJException(String message, int code) {
+    super(message);
+    if (errorCode >= 0 && errorCode < TJ.NUMERR)
+      errorCode = code;
+  }
+
   public TJException(Throwable cause) {
     super(cause);
   }
 
+  /**
+   * Returns a code (one of {@link TJ TJ.ERR_*}) indicating the severity of the
+   * last error.
+   *
+   * @return a code (one of {@link TJ TJ.ERR_*}) indicating the severity of the
+   * last error.
+   */
+  public int getErrorCode() {
+    return errorCode;
+  }
+
+  private int errorCode = TJ.ERR_FATAL;
 }