Improve typegen error handling

[StuartHarris]

Fixes #161

Improves error handling during typegen and adds TypeGenError::PnpmNotFound in order to catch the more common error of not having pnpm installed. The error now looks like this:

cargo build
   Compiling shared_types v0.1.0 (/Users/stuartharris/src/redbadger/crux/examples/simple_counter/shared_types)
error: failed to run custom build command for `shared_types v0.1.0 (/Users/stuartharris/src/redbadger/crux/examples/simple_counter/shared_types)`

Caused by:
  process didn't exit successfully: `/Users/stuartharris/src/redbadger/crux/examples/simple_counter/target/debug/build/shared_types-8703d4b47d26cc47/build-script-build` (exit status: 1)
  --- stdout
  cargo:rerun-if-changed=../shared

  --- stderr
  Error: `pnpm` is needed for TypeScript type generation, but it could not be found in PATH.
  Please install it from https://pnpm.io/installation

  Caused by:
      No such file or directory (os error 2)