Fix Lagrange bases caching on the web

[mitschabaude]

companion of o1-labs/o1js#1906

this fixes a case where compile() got stuck on the web version and kept spinning without error.

diagnosis:

• when the recursive verifier circuit needed access to fp lagrange bases, it ended up in lagrangeCommitment() in srs.ts
• there, in the case of a cache miss, we attempted to call getLagrangeBasis() to get the entire lagrange basis and store it in the cache
• however, in the browser, there is an additional layer behind methods using concurrency, like caml_fq_srs_get_lagrange_basis which getLagrangeBasis() aliases to. this additional layer (see web-backend.js > overrideBindings()) was not able to handle the Uint32Array return value of caml_fq_srs_get_lagrange_basis(). instead, that return value was interpreted in a way that caused the method to never return (even though the lagrange basis had been successfully created!)

the minimal fix is twofold:

1. in the web worker layer, throw an error when calling one of the methods that would never return.
2. in the SRS caching layer, only attempt to return the full LB if we even have a writable cache. otherwise, call a different method that will store the LB internally but not attempt to return it.

The fix circumvents the problem in the most typical case, where we don't have a writable cache in the browser. In the case where we do have a writable cache, the error message and comments added here will make it very obvious what the problem is.

The full solution (which I don't have time for) is outlined in code comments