Collect structured errors from parsing/executing KCL (#187)

Currently, syntax/semantic errors in the user's source code result in vanilla JS exceptions being thrown, so they show up in the console. Instead, this PR:

- Adds a new type KCLError
- KCL syntax/semantic errors when parsing/executing the source code now throw KCLErrors instead of vanilla JS exceptions.
- KCL errors are caught and logged to a new "Errors" panel, instead of the browser console.

src/lang/errors.ts

@@ -0,0 +1,57 @@
+export class KCLError {
+  kind: string | undefined
+  sourceRanges: [number, number][]
+  msg: string
+  constructor(
+    kind: string | undefined,
+    msg: string,
+    sourceRanges: [number, number][]
+  ) {
+    this.kind = kind
+    this.msg = msg
+    this.sourceRanges = sourceRanges
+    Object.setPrototypeOf(this, KCLError.prototype)
+  }
+}
+
+export class KCLSyntaxError extends KCLError {
+  constructor(msg: string, sourceRanges: [number, number][]) {
+    super('syntax', msg, sourceRanges)
+    Object.setPrototypeOf(this, KCLSyntaxError.prototype)
+  }
+}
+
+export class KCLSemanticError extends KCLError {
+  constructor(msg: string, sourceRanges: [number, number][]) {
+    super('semantic', msg, sourceRanges)
+    Object.setPrototypeOf(this, KCLSemanticError.prototype)
+  }
+}
+
+export class KCLTypeError extends KCLError {
+  constructor(msg: string, sourceRanges: [number, number][]) {
+    super('type', msg, sourceRanges)
+    Object.setPrototypeOf(this, KCLTypeError.prototype)
+  }
+}
+
+export class KCLUnimplementedError extends KCLError {
+  constructor(msg: string, sourceRanges: [number, number][]) {
+    super('unimplemented feature', msg, sourceRanges)
+    Object.setPrototypeOf(this, KCLUnimplementedError.prototype)
+  }
+}
+
+export class KCLValueAlreadyDefined extends KCLError {
+  constructor(key: string, sourceRanges: [number, number][]) {
+    super('name', `Key ${key} was already defined elsewhere`, sourceRanges)
+    Object.setPrototypeOf(this, KCLValueAlreadyDefined.prototype)
+  }
+}
+
+export class KCLUndefinedValueError extends KCLError {
+  constructor(key: string, sourceRanges: [number, number][]) {
+    super('name', `Key ${key} has not been defined`, sourceRanges)
+    Object.setPrototypeOf(this, KCLUndefinedValueError.prototype)
+  }
+}