スタックトレース改善

スタックとレースを出来るだけ安全に出力できるように調整。また psyntax 由来でもユーザーフレンドリな情報を表示するように改善。

dorami% ./mosh r6rs-examples/wiki
 Condition components:
   1. &assertion
   2. &who: "cdr"
   3. &message: "pair required"
   4. &irritants: (a)

 Exception:
     error in raise: returned from non-continuable exception

 Stack trace:
    1. throw: <subr>
    2. (raise c):  compiler-with-library.scm:3087
    3. (register-plugin plugin): <unknown location>
    4. eval: <subr>
    5. (invoke-library lib):  psyntax.scm:8
    6. (invoke-library lib):  psyntax.scm:8
    7. (for-each-1 proc lst):  compiler-with-library.scm:1029
    8. (lambda () ...):  psyntax.scm:10
    9. (dynamic-wind in body out):  compiler-with-library.scm:3006
    10. (dynamic-wind in body out):  compiler-with-library.scm:3006
    11. (dynamic-wind in body out):  compiler-with-library.scm:3006
    12. (<top-level>): <unknown location>
    13. (<top-level>): <unknown location>