Don't report problems if origin files do not exist anymore

[jvican]

See https://twitter.com/channingwalton/status/1074592345256017920

[olafurpg]

Steps to reproduce

• create file A.scala
• add compile error to A.scala
• diagnostics are published as expected

{
  "method": "textDocument/publishDiagnostics",
  "params": {
    "uri": "file:///Users/olafurpg/dev/metals/tests/unit/src/test/scala/tests/BspSwitchSlowSuite.scala",
    "diagnostics": [
      {
    ...
        "message": "type mismatch;\n found   : String(\"\")\n required: Int"
      }
    ]
  }
}

• remove A.scala
• expected a publishDiagnostics notification with reset=true and diagnostics=[] for A.scala, but obtained no notification.

[channingwalton]

Awesome, thank you.