fix(auth_login): error handling with invalid instance URL

Context

ggshield auth login flow errors were not handled when instance URL is invalid.

What has been done

Error thrown are handled, similarly to how error are handled with GITGUARDIAN_INSTANCE='https://dashboard.gitguardian.com/abc' ggshield auth login --method=token

Validation

Run the following commands:

GITGUARDIAN_INSTANCE='https://dashboard.gitguardian.com/abc' ggshield auth login
GITGUARDIAN_INSTANCE='https://dashboard.example.com' ggshield auth login

Note

This also fixes the following issue: #892

PR check list

[x] As much as possible, the changes include tests (unit and/or functional)
[x] If the changes affect the end user (new feature, behavior change, bug fix) then the PR has a changelog entry (see doc/dev/getting-started.md). If the changes do not affect the end user, then the skip-changelog label has been added to the PR.

Codecov Report

Attention: Patch coverage is 66.66667% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 91.81%. Comparing base (372be8a) to head (d7f1289). Report is 63 commits behind head on main.

Files	Patch %	Lines
ggshield/cmd/auth/login.py	42.85%	4 Missing :warning:
ggshield/core/url_utils.py	85.71%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #897 +/- ## ========================================== + Coverage 91.67% 91.81% +0.13% ========================================== Files 170 171 +1 Lines 7060 7046 -14 ========================================== - Hits 6472 6469 -3 + Misses 588 577 -11 ``` | [Flag](https://app.codecov.io/gh/GitGuardian/ggshield/pull/897/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/GitGuardian/ggshield/pull/897/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | `91.81% <66.66%> (+0.13%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian#carryforward-flags-in-the-pull-request-comment) to find out more.

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