watch: "Type model errors prevented the JSII assembly.." omits error information

## :bug: Bug Report

### Affected Languages

- [x] `TypeScript` or `Javascript`
- [ ] `Python`
- [ ] `Java`
- [ ] .NET (`C#`, `F#`, ...)

### General Information
* **JSII Version:**
* **Platform:**

### What is the problem?

When using `jsii --watch` I often get this error message without the diagnostics:

```
1:44:14 AM - File change detected. Starting incremental compilation...

[2020-08-27T01:44:15.014] [ERROR] jsii/compiler - Type model errors prevented the JSII assembly from being created

1:44:15 AM - Found 0 errors. Watching for file changes.
```

When running `jsii`, the output includes the diagnostic info:

```
[2020-08-27T01:44:20.852] [ERROR] jsii/compiler - Type model errors prevented the JSII assembly from being created
error TS0: Method and non-static non-readonly property names must use camelCase: DEFAULT_VERSION
```

Reproduce the report by running `jsii --watch` with a TypeScript error such as the `DEFAULT_VERSION` example, then compare its output with a normal `jsii` run. Trace the watch-mode compilation entry point and ensure the diagnostic information is visible; done when watch mode reports the same useful diagnostic as the non-watch command.

Written by the indexing model from the issue text.