error reporting E_ALL switch

  - Adds a constant DOKU_E_LEVEL for globally overriding the error reporting level.
  - Adds a file based switch for enabling E_ALL errors without changing the code.

This is convenient for developers debugging new patches. It also ensures that
the new patch will not accidentally record manual changes to the error reporting
level. To enable E_ALL errors globally, create the file 'conf/report_e_all'.

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