sync(tests): improve error for invalid TOML (#614)

A manually edited `tests.toml` file might contain invalid TOML. For
example:

    [2ee1d9af-1c43-416c-b41b-cefd7d4d2b2a]
    description = encode yes

where `encode yes` is invalid [1] because it is unquoted.

Before this commit, `configlet sync` would not handle the TOML parsing
exception:

    $ cd /tmp
    $ git clone --quiet https://github.com/exercism/common-lisp
    $ cd common-lisp
    $ git checkout f521b1fb0f04ffc2d5baa6cf0bba37c231cc1bd7
    $ bin/fetch-configlet
    $ bin/configlet sync --tests
    Updating cached 'problem-specifications' data...
    Checking exercises...
    parsetoml.nim(908)       parseValue
    Error: unhandled exception: /tmp/common-lisp/exercises/practice/affine-cipher/.meta/tests.toml(6:16) unexpected character "e" [TomlError]

With this commit, configlet still exits immediately, but handles the
exception:

    $ configlet sync --tests
    Updating cached 'problem-specifications' data...
    Checking exercises...

    Error: A 'tests.toml' file contains invalid TOML:
    /tmp/common-lisp/exercises/practice/affine-cipher/.meta/tests.toml(6:16) unexpected character "e"

    The expected 'tests.toml' format is documented in
    https://exercism.org/docs/building/configlet/sync#h-tests

Note that `configlet lint` does not yet lint `tests.toml` files, and so
invalid TOML does not yet cause `configlet lint` to indicate an error.

[1] https://toml.io/en/v1.0.0#string

Fixes: #613

src/sync/tracks.nim

@@ -1,4 +1,4 @@
-import std/[algorithm, json, os, sets]
+import std/[algorithm, json, os, sets, strformat, strutils]
 import pkg/parsetoml
 import ".."/cli
 
@@ -65,7 +65,21 @@ proc init(T: typedesc[PracticeExerciseTests], testsPath: string): T =
   ## included and excluded test case UUIDs.
   result = PracticeExerciseTests.init()
   if fileExists(testsPath):
-    let tests = parsetoml.parseFile(testsPath)
+    let tests =
+      try:
+        parsetoml.parseFile(testsPath)
+      except TomlError: # Note that `TomlError` inherits from `Defect`.
+        stderr.writeLine fmt"""
+
+          Error: A 'tests.toml' file contains invalid TOML:
+          {getCurrentExceptionMsg()}
+
+          The expected 'tests.toml' format is documented in
+          https://exercism.org/docs/building/configlet/sync#h-tests""".unindent()
+        quit 1
+      except CatchableError:
+        stderr.writeLine "Error: " & getCurrentExceptionMsg()
+        quit 1
 
     for uuid, val in tests.getTable():
       if val.hasKey("include"):