Server should not start in case of incorrectly formated json file
I have a typo in my configuration file:
```
[WARNING] [10:13:23] {SESSION MANAGER} - session_manager.py:149 load_session_cfg() - No JSON object could be decoded
[WARNING] [10:13:23] {SESSION MANAGER} - session_manager.py:151 load_session_cfg() - Not valid user authentication file: /var/lib/jenkins/.codechecker/session_config.json

```

In that case, the server should not start:
* it is risky 
* this is clearly not what I want when I am making a mistake in the configuration

diff --git a/libcodechecker/server/server.py b/libcodechecker/server/server.py
index 4b024e47..16661538 100644
--- a/libcodechecker/server/server.py
+++ b/libcodechecker/server/server.py
@@ -19,6 +19,7 @@ from random import sample
 import stat
 import socket
 import ssl
+import sys
 import urllib
 
 try:
@@ -748,6 +749,12 @@ def start_server(config_directory, package_data, port, db_conn_string,
                      .format(root_file))
             root_sha = __make_root_file(root_file)
 
+    try:
+        manager = session_manager.SessionManager(root_sha, force_auth)
+    except IOError, ValueError:
+        LOG.error("The server's authentication config file is invalid!")
+        sys.exit(1)
+
     http_server = CCSimpleHttpServer(server_addr,
                                      RequestHandler,
                                      config_directory,
@@ -757,9 +764,7 @@ def start_server(config_directory, package_data, port, db_conn_string,
                                      suppress_handler,
                                      context,
                                      check_env,
-                                     session_manager.SessionManager(
-                                         root_sha, force_auth)
-                                     )
+                                     manager)
 
     try:
         instance_manager.register(os.getpid(),
diff --git a/libcodechecker/session_manager.py b/libcodechecker/session_manager.py
index 7e074e01..95580992 100644
--- a/libcodechecker/session_manager.py
+++ b/libcodechecker/session_manager.py
@@ -145,10 +145,12 @@ def load_session_cfg(session_cfg_file):
     except IOError:
         LOG.debug('Failed to open user authentication file: ' +
                   session_cfg_file)
+        raise
     except ValueError as verr:
         LOG.warning(verr)
         LOG.warning('Not valid user authentication file: ' +
                     session_cfg_file)
+        raise
 
     return scfg_dict
 
@@ -442,7 +444,11 @@ class SessionManager_Client:
                                         ".codechecker.passwords.json")
         LOG.debug(session_cfg_file)
 
-        scfg_dict = load_session_cfg(session_cfg_file)
+        try:
+            scfg_dict = load_session_cfg(session_cfg_file)
+        except (IOError, ValueError):
+            # On the client's side, continue execution with defaults.
+            scfg_dict = {}
 
         if not scfg_dict.get("credentials"):
             scfg_dict["credentials"] = {}
['libcodechecker/session_manager.py', 'libcodechecker/server/server.py']