`try-toolchain` crashes EasyBuild if toolchain is not known

[Thyre]

Easy one to reproduce:

$ eb --try-toolchain=impi,2025b Valgrind-3.25.1-gompi-2025b.eb
== Temporary log file in case of crash /tmp/eb-r73kbz6u/easybuild-8a9rczzk.log
EasyBuild crashed! Please consider reporting a bug, this should not happen...

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/EasyBuild/prog/local/lib/python3.12/dist-packages/easybuild/main.py", line 855, in <module>
    main_with_hooks()
  File "/opt/EasyBuild/prog/local/lib/python3.12/dist-packages/easybuild/main.py", line 841, in main_with_hooks
    main(args=args, prepared_cfg_data=(init_session_state, eb_go, cfg_settings))
  File "/opt/EasyBuild/prog/local/lib/python3.12/dist-packages/easybuild/main.py", line 793, in main
    do_cleanup = process_eb_args(orig_paths, eb_go, cfg_settings, modtool, testing, init_session_state,
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/EasyBuild/prog/local/lib/python3.12/dist-packages/easybuild/main.py", line 475, in process_eb_args
    easyconfigs, tweak_map = tweak(easyconfigs, build_specs, modtool, targetdirs=tweaked_ecs_paths, return_map=True)
                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/EasyBuild/prog/local/lib/python3.12/dist-packages/easybuild/framework/easyconfig/tweak.py", line 146, in tweak
    src_to_dst_tc_mapping = map_toolchain_hierarchies(source_toolchain, target_toolchain, modtool)
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/EasyBuild/prog/local/lib/python3.12/dist-packages/easybuild/framework/easyconfig/tweak.py", line 840, in map_toolchain_hierarchies
    target_tc_hierarchy = get_toolchain_hierarchy(target_toolchain, incl_capabilities=True)
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/EasyBuild/prog/local/lib/python3.12/dist-packages/easybuild/framework/easyconfig/easyconfig.py", line 234, in cache_aware_func
    toolchain_hierarchy = func(toolchain, incl_capabilities)
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/EasyBuild/prog/local/lib/python3.12/dist-packages/easybuild/framework/easyconfig/easyconfig.py", line 335, in get_toolchain_hierarchy
    subtoolchain_names = subtoolchains[current_tc_name]
                         ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
KeyError: 'impi'

Trying a toolchain which does not exist, e.g. impi (easy typo), EasyBuild crashes. Instead, it should complain that the toolchain does not exist.