Avoid unrequired `copen`

[LucHermitte]

BTW used to execute copen in foreground compilation in order to know whether errors have occurred. In background compilation (through Vim 8 +job), we have the information though other means. In consequence, we shouldn't need to execute copen if the qf window is already opened -- executing it triggers pointless events.

However, lh#btw#build#_show_error() is also executed in order to:

• resize the qf window,
• apply syntax highlighting to qf buffer,
• apply syntax hooks to qf buffer/window

These features may be required. May be, the first one could be achieved though other means. For the other ones, the BTW-filters will need to register whether they are doing anything -- some may even execute filename/message filtering on the fly (in btw/job_build.vim s:callbackCB())