Skip candidates with invalid theme() calls

[thecrypticace]

Right now when we encounter a candidates with invalid theme() calls we throw an error which stops the build entirely. This is not ideal because, especially in the case of node_modules, if one file in one package has an error it will stop the build for an entire project and tracking this down can be quite difficult.

Now, after this PR, any candidates that use theme(…) with non-existent theme keys (e.g. rounded-[theme(--radius-does-not-exist)]) will be skipped instead of breaking the build.

Before:

<div class="underline rounded-[theme(--radius-does-not-exist)]"></div>

/* No CSS was generated because an error was thrown */
/* Error: Invalid theme key: --radius-does-not-exist */

After:

<div class="underline rounded-[theme(--radius-does-not-exist)]"></div>

.underline {
  text-decoration-line: underline;
}