feat(log): Improve error logging (refs #5080)

HTTP responses' errors are now logged with PHP's error_log default
output (i.e. Apache error.log for Web, and STDERR for CLI) + syslog.

Syslog messages are sent with identity/tag "dynacase-control", facility
LOG_USER and varying priorities (LOG_ERR, LOG_INFO, etc.).

Additionally, if the "Debug mode" is activated in the Web UI, then
errors are also logged in a `$WIFF_ROOT/conf/wiff.log` file. A set of
"View" and "Clear" buttons are available for respectively viewing the
file (as plain text) and clearing it's content.

Also, some system() calls have been rewritten with exec() in order to
capture a more complete output of the commands' errors.
feat(CLI): Detailed message for unmet dependencies (refs #4816)