Not great build errors when directives fail

[valencik]

We currently have a couple of directives that can throw errors, and because we are throwing within DirectivePart#map instead of returning a Left in DirectivePart#evalMap (directives.scala) we don't get nice error messages.

If we break some LaTeX in a blog post somewhere, we get:

ParseError: ParseError: KaTeX parse error: Undefined control sequence: \meow at position 6:  (G, \̲m̲e̲o̲w̲)
  at <js> ParseError(Unnamed:108:3648-3663)
  at <js> parseGroup(Unnamed:18595:607334-607402)
  at <js> parseAtom(Unnamed:18023:591421-591461)
  at <js> parseExpression(Unnamed:17877:587964-587995)
  at <js> parse(Unnamed:17809:585880-585906)
  at <js> parseTree(Unnamed:18829:614302-614315)
  at <js> renderToDomTree(Unnamed:18939:617371-617405)
  at <js> renderToString(Unnamed:18900:616258-616293)
  at org.graalvm.polyglot.Value.invokeMember(Value.java:1159)
  at KaTeX$.apply(build.scala:448)
  at LaikaCustomizations$Directives$.$init$$$anonfun$7(build.scala:290)

A good error message from KaTeX, but with no additional context from Laika.

This also applies to our svg directive.