Microdraw will no longer force quit when essential files are missing

[xgui3783]

This PR aims to allow deveopers and users who want to tryout Microdraw to

git clone
npm i
npm start

and have an instance of Microdraw running without copy pasting the required files.

The current behaviour is that the process will exit if any of the files are absent. The PR behaviour is Microdraw will then use the default values (those present in .example).

---

• screenshot_test_walk generates the same test pictures as there were and does not show any errors
• These changes fix program hard fails when any of the following files are missing: server_config.json, github-keys.json, configuration.json, base.js #99 (github issue number if applicable).
• All MicroDraw tools behave as expected:
  • GESTURES
    • zoom in and out with two finger drag works
  • KEYS
    • right and up arrow key or left and down arrow key
      • jump to next or previous slice respectively
      • update the slider accordingly
      • update the slice number accordingly
    • cmd z undoes
      • any step back that you have done in their chronological order including
      • translation of region
      • drawing region
      • adding point
      • deleting point
        to be completed (maybe it is implemented and working for all functions, so we can delete the single step checks)
  • TOOL BUTTONS
    • red rectangle in navigation window
      • corresponds to tissue piece selected in viewer
      • can be navigated upon mouse down drag
    • navigation tool
      • navigates the slice inside the viewer upon mouse down drag
    • home button
      • zooms out to view the data in full screen size view
    • zoomIn tool
      • zooms one step in upon click
    • zoomOut tool
      • zooms one step out upon click
    • left arrow
      • jumps to the previous slice
    • right arrow
      • jumps to the next slice
    • select tool
      • selects a region upon click
    • draw tool
      • draws a new region as bezier curve
    • drawPolygon tool
      • draws a new region as a polygon path
    • simplify tool
      • decreases the number of points in the region path while aiming at conserving the shape
    • addPoint tool
      • adds a point to the path upon click at position of mouse click
    • deletePoint tool
      • deletes a point from the path upon mouse click
    • addRegion tool
      • unifies two overlapping regions into one
    • deleteRegion tool
      • deletes a region upon click
    • splitRegion tool
      • splits one region at the intersection path with a second region
    • rotate tool
      • rotates a region around the point of mouse click
    • flip tool
      • flips a region around its vertical axis
    • toPolygon tool
      • converts the region path from bezier curve to polygon path
    • toBezier
      • converts the region path from polygon path to bezier curve
    • copy tool
      • copies a region
    • paste tool
      • pastes the copied region
    • save tool
      • saves the set of drawn regions to the database
    • screenshot tool
      • creates a screenshot of the data layer (not of the annotations yet) at a chosen resolution
    • delete tool
      • deletes the selected region
    • closeMenu tool
      • hides the menu bar upon click
    • openMenu tool
      • shows the menu bar upon click
    • undo tool
      • undoes the user's actions in reverse chronological order

• I implemented tests for these changes OR
• These changes do not require tests because _____

[xgui3783]

merged into #112