Plugin failures should not take down or make hang HLS

I encounter a case where entire server is crashed by a failed parse in `Eval` plugin:

```
haskell-language-server: Tokens.next failed to parse
{- else -} (structLit Parser did not consume entire stream, left:>
CallStack (from HasCallStack):
error, called at src/Ide/Plugin/Eval/Parse/Token.hs:134:19 in hls-eval-plugin-0.1.0.0-2Df7iIOQIttFUpUdbeyzFM:Ide.Plugin.Eval.Parse>
```

The code that fails to parse looks more or less like this:

```haskell
expr =
let_
1 {- = -} (setField "foo" (stringLit "hello") (v 0)) {- in -} $
let_
2 {- = -} (setField "bar" (stringLit "world") (v 1)) {- in -} $
let_
3 {- = -} (AST.ExprIf (AST.ExprFunction (AST.StringIsEmpty (getField "foo" (v 2))))
{- then -} (setField "bar" (stringLit "hello world") (v 2))
{- else -} (structLit
[ "foo" .= StringValue "foo"
, "bar" .= StringValue "bar"
, "baz" .= StringValue "baz"
])) {- in -} $
v 3
```

GHC has no problems with it.

In my humble opinion, the problem is not that this piece fails to parse for whatever reason but rather in that it crashes HLS.
Hope it helps! Thank you in advance

### Your environment
Output of `haskell-language-server --probe-tools` or `haskell-language-server-wrapper --probe-tools`:

```sh
haskell-language-server version: 0.8.0.0 (GHC: 8.8.4) (PATH: /nix/store/3dkwi0xnifx6882f1czw9x7hsv63qj0x-haskell-language-server-0.8.0.0/bin/haskell-language-server)
Tool versions found on the $PATH
cabal: Not found
stack: 2.5.1
ghc: 8.10.3
```
Which lsp-client do you use:
VS Codium
Describe your project (alternative: link to the project):
N/A
Contents of `hie.yaml`: N/A

### Steps to reproduce

In general, you have to find a piece that triggers the above behavior in Eval plugin and include it in a project. Judging by the logs, I'd say that it's caused by keywords in comments (but that does not seem right). You can, of course use, the one I provided (in case it didn't reproduce, I could try to find a minimal reproducer)
### Expected behaviour

HLS should not crash (regardless of whether a piece of code can be parsed by the plugin or not)
### Actual behaviour

HLS crashes - VS displays pop-up that it crashed more than x times and won't be restarted
### Include debug information
Execute in the root of your project the command `haskell-language-server --debug .` and paste the logs here:

Debug output:

```

```

Paste the logs from the lsp-client, e.g. for [VS Code](https://github.com/haskell/vscode-haskell#troubleshooting)

LSP logs:

```
haskell-language-server version: 0.8.0.0 (GHC: 8.8.4) (PATH: /nix/store/3dkwi0xnifx6882f1czw9x7hsv63qj0x-haskell-language-server-0.8.0.0/bin/haskell-language-server)
Starting (haskell-language-server)LSP server...
with arguments: LspArguments {argLSP = True, argsCwd = Nothing, argFiles = [], argsShakeProfiling = Nothing, argsTesting = False, argsExamplePlugin = False, argsDebugOn = False, argsLogFile = Nothing, argsThreads = 0, argsProjectGhcVersion = False}
with plugins: [PluginId "brittany",PluginId "class",PluginId "eval",PluginId "floskell",PluginId "ghcide",PluginId "hlint",PluginId "importLens",PluginId "moduleName",PluginId "ormolu",PluginId "pragmas",PluginId "retrie",PluginId "stylish-haskell",PluginId "tactic"]
in directory: /home/marcin/channable/sharkmachine
If you are seeing this in a terminal, you probably should have run ghcide WITHOUT the --lsp option!
Started LSP server in 0.00s
2021-01-19 14:37:26.487612469 [ThreadId 25] - Registering ide configuration: IdeConfiguration {workspaceFolders = fromList [NormalizedUri (-8084739005052116551) "file:///home/marcin/channable/sharkmachine"], clientSettings = hashed Nothing}
2021-01-19 14:37:26.48896058 [ThreadId 25] - Configuration changed: Object (fromList [("haskell",Object (fromList [("logFile",String ""),("updateBehavior",String "never-check"),("hlintOn",Bool True),("formatOnImportOn",Bool True),("indentationRules",Object (fromList [("enabled",Bool True)])),("liquidOn",Bool False),("languageServerVariant",String "haskell-language-server"),("serverExecutablePath",String "${workspaceFolder}/contrib/haskell-language-server.sh"),("diagnosticsOnChange",Bool True),("completionSnippetsOn",Bool True),("maxNumberOfProblems",Number 30.0),("formattingProvider",String "stylish-haskell"),("trace",Object (fromList [("server",String "off")]))]))])

...

2021-01-19 14:37:44.144813922 [ThreadId 113] - Using interface files cache dir: ghcide
2021-01-19 14:37:44.144874525 [ThreadId 113] - Making new HscEnv[main]

...

2021-01-19 14:37:44.992944709 [ThreadId 1992] - finish: CodeAction (took 0.00s)
2021-01-19 14:37:45.73748366 [ThreadId 2427] - finish: codeLens (took 1.28s)
2021-01-19 14:37:45.738428253 [ThreadId 2433] - finish: (took 0.00s)
2021-01-19 14:37:45.739461973 [ThreadId 2434] - finish: ModuleName.ghcSession (took 0.00s)
2021-01-19 14:37:45.739877384 [ThreadId 2436] - finish: ModuleName.GetParsedModule (took 0.00s)
haskell-language-server: Tokens.next failed to parse {- else -} (structLit Parser did not consume entire stream, left: "(structLit"
CallStack (from HasCallStack):
error, called at src/Ide/Plugin/Eval/Parse/Token.hs:134:19 in hls-eval-plugin-0.1.0.0-2Df7iIOQIttFUpUdbeyzFM:Ide.Plugin.Eval.Parse.Token
2021-01-19 14:37:45.925001278 [ThreadId 2460] - finish: CodeAction:PackageExports (took 0.93s)
2021-01-19 14:37:45.925005508 [ThreadId 2459] - finish: CodeAction:PackageExports (took 1.36s)
2021-01-19 14:37:45.92524896 [ThreadId 2461] - finish: importLens (took 0.00s)
2021-01-19 14:37:45.925462936 [ThreadId 2463] - finish: addPragma (took 0.00s)
2021-01-19 14:37:45.925708555 [ThreadId 2465] - finish: retrie (took 0.00s)
2021-01-19 14:37:45.925910353 [ThreadId 2467] - finish: tactic (took 0.00s)
2021-01-19 14:37:45.926165106 [ThreadId 2472] - finish: tactic (took 0.00s)
2021-01-19 14:37:45.926210695 [ThreadId 2473] - finish: tactic (took 0.00s)
[Info - 2:37:46 PM] Connection to server got closed. Server will restart.
[Error - 2:37:46 PM] Request textDocument/codeLens failed.
Error: Connection got disposed.
at Object.dispose (/home/marcin/.vscode/extensions/haskell.haskell-1.2.0/dist/extension.js:1:90964)

```

Start in src/Ide/Plugin/Eval/Parse/Token.hs around line 134 and reproduce the failure with the supplied Eval plugin snippet. Trace how the parser failure reaches the HLS process, then verify that the same invalid input reports a plugin parse failure without crashing or hanging the server.

Written by the indexing model from the issue text.