🚸 Improve error message for cyclic flows

[elbeno]

The error trace given is something like:

/.../include/flow/graph_builder.hpp:150:27: fatal error: static assertion failed due to requirement 'built.has_value()': Topological so
rt failed: cycle in flow
  150 |             static_assert(built.has_value(),
      |                           ^~~~~~~~~~~~~~~~~
/.../include/flow/graph_builder.hpp:155:39: note: in instantiation of member function 'flow::graph_builder<flow::impl>::built_flow<cib:
:initialized<(anonymous namespace)::CyclicFlowConfig, (anonymous namespace)::TestFlowAlpha>>::built' requested here
  155 |         constexpr static auto run() { built()(); }
      |                                       ^
/.../include/flow/graph_builder.hpp:160:53: note: in instantiation of member function 'flow::graph_builder<flow::impl>::built_flow<cib:
:initialized<(anonymous namespace)::CyclicFlowConfig, (anonymous namespace)::TestFlowAlpha>>::run' requested here
  160 |         constexpr auto operator()() const -> void { run(); }
      |                                                     ^
/.../test/flow/fail/cyclic_flow.cpp:22:33: note: in instantiation of member function 'flow::graph_builder<flow::impl>::built_flow<cib::
initialized<(anonymous namespace)::CyclicFlowConfig, (anonymous namespace)::TestFlowAlpha>>::operator()' requested here
   22 |     nexus.service<TestFlowAlpha>();
      |                                 ^

So the flow that fails (TestFlowAlpha) is in the output just after the static_assert message.