[READY] Fix diagnostic highlighting with invalid location extent
ycmd may return a location extent with a starting column of zero (see the test for an example) which is not a valid column (columns are 1-indexed). We should use the location in this case. Also, when the location extent is not valid, we should call `AddDiagnosticSyntaxMatch` with the `is_error` parameter to highlight warnings as such and not as errors (`is_error` is `True` by default).
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/valloric/youcompleteme/2602)
<!-- Reviewable:end -->
On non-Windows platforms, a ConnectionError exception is raised instead
of a ConnectTimeout one when the connection cannot be established (e.g.
the server crashed). Since the latter is a subclass of the former, we
only need to catch ConnectionError.
Sitecustomize files can be slow or broken and might hang editor
startup. Since they aren't necessary for checking the python version,
disable them to get a small speed boost for everyone, and an editor
that doesn't hang indefinitely on startup if the sitecustomize gets
into an infinite loop (due to a bad NFS mount or similar).
[READY] Fix exception when response future is not set
An `AttributeError` exception is raised when `Done` is called before `Start` in the `CompletionRequest` and `EventNotification` classes because the `_response_future` attribute is not yet defined.
Fixes#2461.
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/valloric/youcompleteme/2465)
<!-- Reviewable:end -->
[READY] Catch and log all server exceptions
A lot of different errors may occur when sending a request to ycmd and receiving its response:
- [any exception from the requests module](http://docs.python-requests.org/en/master/_modules/requests/exceptions/);
- exceptions from ycmd: `ServerError` and `UnknownExtraConf`;
- `RuntimeError` exception from invalid HMAC;
- and possibly others.
Thanks to PR #2430, we can now catch and log these exceptions.
I am marking this PR as WIP because I'd like to add tests but I am not sure on how to implement them. Should we use actual code for these tests or mock the response from the ycmd server?
Fixes#2216.
Fixes#2272.
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/valloric/youcompleteme/2453)
<!-- Reviewable:end -->
[READY] Fix flake8 error
This fixes a new error detected by the last version of `pycodestyle` (2.2.0), which is a dependency of `flake8`:
```
C:\projects\youcompleteme\python\ycm\youcompleteme.py:69:1: E305 expected 2 blank lines after class or function definition, found 1
```
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/valloric/youcompleteme/2441)
<!-- Reviewable:end -->
