Error messages are swallowed

[Prinzhorn]

I'm not sure what the root cause is, maybe this needs to be fixed in prettier or svelte, but I'm not familiar enough with their architecture. You might know immediately where this belongs.

As mentioned in #70 I wasn't getting any meaningful error message. All I got was

[error] Unable to expand glob patterns: js/components/DetachableWindow.svelte !**/node_modules/** !./node_modules/** !**/.{git,svn,hg}/** !./.{git,svn,hg}/**
[error] Cannot read property 'line' of undefined

I was able to track this down to node_modules/svelte/compiler.js

	class CompileError extends Error {
	    toString() {
	        return `${this.message} (${this.start.line}:${this.start.column})\n${this.frame}`;
	    }
	}

where this.start is undefined. It should be this.loc.start. But I don't think something that obvious is a bug in svelte. Not sure what's going on here, since this is compiled from TypeScript.

Once I fixed the line I got

[error] js/components/DetachableWindow.svelte: <script> must have a closing tag
[error] > 1 | <script>
[error]     |        ^
[error]   2 |   import { onDestroy } from "svelte";
[error]   3 | 
[error]   4 |   let wrapperRef; (1:8)
[error] 1: <script>
[error]            ^
[error] 2:   import { onDestroy } from "svelte";
[error] 3: 

which helped a lot more.

[ehrencrona]

I could reproduce this and I agree with the analysis above. The start attribute on the error is removed in https://github.com/sveltejs/prettier-plugin-svelte/blob/master/src/index.ts#L33
I don't quite understand why that code does what it does though. We could change it to make an exception for CompileError, but it's quite possible it can just be removed.

@dummdidumm Do you know?

[dummdidumm]

Honestly I don't know, I only started taking over the project recently. This PR added it, and it seems it did that for a reason, but I guess we can just delete the delete (hah).