Fix diagnostics printing when `pwd()` doesn't exist

[c42f]

Computing the file URL for pretty printing using abspath can fail when the current working directory doesn't exist. This is a quick fix for this issue (which is incredibly confusing/disruptive if the user does manage to enter a nonexistant working directory!)

A more complete fix would avoid ever looking at the working directory when printing diagnostics, instead requiring the caller to pass in a richer definition of the "location of source code" than the mere file name as a string. However getting something sensible working there is (a) breaking and (b) unclear on may details. So just patching this up quickly seems good for now.

Fix #359

[codecov[bot]]

Codecov Report

Merging #373 (4ac765c) into main (1b048aa) will decrease coverage by 0.03%. The diff coverage is 90.00%.

@@            Coverage Diff             @@
##             main     #373      +/-   ##
==========================================
- Coverage   96.58%   96.56%   -0.03%     
==========================================
  Files          14       14              
  Lines        4160     4164       +4     
==========================================
+ Hits         4018     4021       +3     
- Misses        142      143       +1     

Files	Coverage Δ
src/diagnostics.jl	93.18% <90.00%> (-1.82%)	:arrow_down: