Improved error handling for message search method

[ikarol]

closes #444

I wasn't able to reproduce(through tests) the issue reported in #444 as it's server specific and not trivial. However, this change doesn't break existing tests for search functionality.

[Slamdunk]

See https://github.com/ddeboer/imap/pull/436/files#r370508077

[ikarol]

See https://github.com/ddeboer/imap/pull/436/files#r370508077

Hi,
there is one key difference in my use case and implementation vs #436
In my use case \imap_search() and \imap_sort() weren't returning false, but empty array(implying that the operation has been executed successfully), so no exception was thrown(and if you were to merge #436 , it wouldn't catch that as well for the same reason) and this error emitted notice at the end(see #444 for description) that broke my script. This was the error caused by mailbox missconfiguration.

So, my goal is to catch this kind of errors and prevent them from producing incorrect results. Now, I understand that such errors might not be relevant to InvalidSearchCriteriaException and we might need to introduce new exception type which will be responsible for errors caused "in between" search operation.

What do you think?

[Slamdunk]

Ok, if so please use false !== \imap_last_error() instead of imap_errors(): imap_last_error doesn't alter the error stack, so the exceptions can stay the same

[ikarol]

Do you think we need a new exception type for this to separate it from search criteria specific cases?

[Slamdunk]

It is a good idea, but not for now it's good as is

[Slamdunk]

Thank you!