[errors-] show callstack above exception catch level #2586

The extra stacktrace levels are helpful when the exception
is caught inside a common function like wrapply() that is called
from many places.

visidata/errors.py

@@ -1,4 +1,5 @@
 import traceback
+import re
 
 from visidata import vd, VisiData
 
@@ -11,8 +12,23 @@ class ExpectedException(Exception):
 
 
 def stacktrace(e=None):
+    '''Return a list of strings. Includes extra callstack levels above the
+    level where the exception was technically caught, to aid debugging.'''
+
     if not e:
-        return traceback.format_exc().strip().splitlines()
+        stack = ''.join(traceback.format_stack()).strip().splitlines()
+        trim_levels = 3   # calling function -> stacktrace() -> format_stack()
+        trace_above = stack[:-2*trim_levels]
+        trace_above[0] = '  ' + trace_above[0]  #fix indent level of first line
+        try:
+            # remove several levels of uninformative stacktrace in typical interactive vd
+            idx = trace_above.index('    ret = vd.mainloop(scr)')
+            trace_above = trace_above[idx+1:]
+        except ValueError:
+            pass
+        # remove lines that mark error columns with carets and sometimes tildes
+        trace_below = [ line for line in traceback.format_exc().strip().splitlines() if not re.match('^ *~*\\^+$', line) ]
+        return [trace_below[0]] + trace_above + trace_below[1:]
     return traceback.format_exception_only(type(e), e)