エラー画面の改善

[nanasess]

• html/error.php は PHP5.1以下で使用されていたファイルなので削除する
  • see https://github.com/EC-CUBE/ec-cube2/blob/master/data/class/helper/SC_Helper_HandleError.php#L59-L67
• /preview はフロント側のエラー画面を表示する

[codecov-commenter]

Codecov Report

Merging #463 (60d151d) into master (86812bb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #463   +/-   ##
=======================================
  Coverage   53.89%   53.89%           
=======================================
  Files          76       76           
  Lines        8992     8992           
=======================================
  Hits         4846     4846           
  Misses       4146     4146           

Flag	Coverage Δ
tests	53.89% <ø> (ø)

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[nanasess]

PHPStan のエラーは #464 で対応

[okazy]

ありがとうございます。マージしました。