Add a StructLike class that extends Error.

Exception classes should extend this so that they derive from Error.

As mentioned in [the GitHub issue](creditkarma/thrift-typescript#178), this is useful because it causes Thrift exceptions to have stack traces and because some frameworks expect exceptions to derive from Error. The GitHub issue mentions graphql-js. Jest's `toThrow()` function would also benefit from this. Here's an example:

```
await expect(thriftClient.someFunction()).rejects.toThrow()
```

`toThrow` doesn't identify Thrift exceptions as exceptions because they don't derive from Error and so it will return false in such cases. Part of the blame could fall on Jest, because perhaps `toThrow` should return true for any rejected promise regardless of the type of object being throw, but I think some of the blame still falls on Thrift.

Possible remaining work:
- Add a test?
- Bump version number so end users can require the new version.

packages/thrift-server-core/src/main/types.ts

@@ -86,6 +86,22 @@ export abstract class StructLike implements IStructLike {
     public abstract write(output: TProtocol): void
 }
 
+/**
+ * Like `StructLike` but extends `Error`. Exception classes extend this.
+ */
+export abstract class ErrorStructLike extends Error implements IStructLike {
+    public readonly name: string
+    public readonly _annotations: IThriftAnnotations = {}
+    public readonly _fieldAnnotations: IFieldAnnotations = {}
+
+    constructor() {
+        super()
+        this.name = this.constructor.name
+    }
+
+    public abstract write(output: TProtocol): void
+}
+
 export interface IStructConstructor<T extends StructLike> {
     new (args?: any): T
     read(input: TProtocol): T