[REGRESSION] file.managed: state.apply "'Changes' should be a dictionary" error and unuseful output if invalid 'source' file path is specified

Description of Issue

Hello!

In Salt 2019.2.2 (and likely any 2019.2.x version), when using the file.managed state and specifying a source file that doesn’t actually exist (e.g. source: salt://path/to/nonexistent/file.txt), state.apply will return a "'Changes' should be a dictionary" error instead of returning the real error, i.e. that I’m trying to use a file that doesn’t exist in SaltFS. This is a break from 2018.3.x which actually reports a useful Source file not found error.

Setup

[root@localhost salt]# pwd
/tmp/salt

[root@localhost salt]# ls
state.sls

[root@localhost salt]# cat state.sls
/tmp/salt/target_file.txt:
  file.managed:
    - source: salt://not_a_file.txt

Steps to Reproduce Issue

Output when using Salt 2019.2.2:

[root@localhost salt]# salt-call --local --file-root=/tmp/salt/ state.apply state test=True
[ERROR   ] (False, u"Source file salt://not_a_file.txt not found in saltenv 'base'")
local:
The State execution failed to record the order in which all states were executed. The state return missing data is:
{u'changes': {},
 u'comment': u"An exception occurred in this state: 'Changes' should be a dictionary.",
 u'name': u'later',
 u'result': False}
----------
          ID: /tmp/salt/target_file.txt
    Function: file.managed
      Result: False
     Comment: An exception occurred in this state: 'Changes' should be a dictionary.
     Changes:

Summary for local
------------
Succeeded: 0
Failed:    1
------------
Total states run:     1
Total run time:   0.000 ms

When using Salt 2018.3.4 the output is much more useful:

[root@localhost salt]# salt-call --local --file-root=/tmp/salt/ state.apply state test=True
[ERROR   ] Source file salt://not_a_file.txt not found
local:
----------
          ID: /tmp/salt/target_file.txt
    Function: file.managed
      Result: False
     Comment: Source file salt://not_a_file.txt not found
     Started: 11:00:08.781896
    Duration: 26.166 ms
     Changes:   Invalid Changes data: (False, u'Source file salt://not_a_file.txt not found')

Summary for local
------------
Succeeded: 0 (changed=1)
Failed:    1
------------
Total states run:     1
Total run time:  26.166 ms

Versions Report

Salt Version:
           Salt: 2019.2.2

Dependency Versions:
           cffi: 1.6.0
       cherrypy: Not Installed
       dateutil: 1.5
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.7.2
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.5.6
   mysql-python: Not Installed
      pycparser: 2.14
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.5 (default, Aug  7 2019, 00:51:29)
   python-gnupg: Not Installed
         PyYAML: 3.11
          PyZMQ: 15.3.0
           RAET: Not Installed
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.2.1
            ZMQ: 4.1.4

System Versions:
           dist: centos 7.7.1908 Core
         locale: UTF-8
        machine: x86_64
        release: 3.10.0-1062.4.1.el7.x86_64
         system: Linux
        version: CentOS Linux 7.7.1908 Core