RollupCompiler does not output warnings in dev mode

[benmccann]

Describe the bug
Sapper doesn't flush Rollup warnings when it receives an error in dev mode

I think these warnings only apply to errors in the rollup.config.js file

To Reproduce
git clone git@github.com:babichjacob/sapper-typescript-graphql-template.git

Pass

{
	files: ["not-exist.ts"],
}

as options to typescript(/* here */). You get the opaque error

@rollup/plugin-typescript: Couldn't process compiler options

Expected behavior

(typescript plugin) @rollup/plugin-typescript TS5023: Unknown compiler option 'files'.

Information about your Sapper Installation:

• Sapper version 0.27.15
• dev mode
• Rollup

Severity
Quite frustrating for users who don't know about this bug. Not so bad for me since I know it exists and can work around it in my rollup config or by running npm run build instead

Additional context
Thanks to @babichjacob for the testing. He discovered it in babichjacob/sapper-typescript-graphql-template#7

#1236 flushed warnings in compile. I didn't realize that dev mode might not call compile. It looks to me like it instead calls a different method watch. We should flush warnings in that case as well. I got most of the last fix from Rollup's cli. Here's what they do: https://github.com/rollup/rollup/blob/08a6255ce0af2dca775e828919f035b433b35fed/cli/run/watch-cli.ts#L93

[benmccann]

cli.ts listens for events: https://github.com/sveltejs/sapper/blob/master/src/cli.ts#L94

These are custom Sapper events output by dev.ts:

sapper/src/api/dev.ts

Line 426 in 71d47b0

compiler.watch((error?: Error, result?: CompileResult) => {

Which are originally from here:

sapper/src/core/create_compilers/RollupCompiler.ts

Line 100 in 9f8e410

case 'FATAL':

[benmccann]

Rollup has a step for parsing the config file that would generate the warnings that we seem not to have:

https://github.com/rollup/rollup/blob/08a6255ce0af2dca775e828919f035b433b35fed/cli/run/watch-cli.ts#L58