Revert error coloring (for development on dedicated branch) (#565)

pkl-cli/src/main/kotlin/org/pkl/cli/CliTestRunner.kt

@@ -89,7 +89,6 @@ constructor(
         } catch (ex: Exception) {
           errWriter.appendLine("Error evaluating module ${moduleUri.path}:")
           errWriter.write(ex.message ?: "")
-          errWriter.write(ex.stackTraceToString())
           if (moduleUri != sources.last()) {
             errWriter.appendLine()
           }