fix(e10): --e10 passed bare silently skipped the controller

Three bugs, all found by actually running the thing rather than reading it.

1. `--e10` with no value did nothing. Making the flag optional-valued gave it
   an empty-string constant, and the code deciding whether to drive the
   controller used a truth test - so the documented `--e10` form printed the
   full translation and quietly skipped the hardware. No error, because
   nothing failed. Now gated on `is not None`, with a comment saying why.

2. A missing part program dumped a traceback. The top-level handler tidied
   e10 errors and re-raised everything else. Now a clean message that lists
   the part programs it can actually see, which also covers Windows path
   confusion.

3. Errors could vanish when piped. stdout is block-buffered and stderr is
   not, so the message printed before the output it referred to and a `| tail`
   dropped it entirely - looking like a silent failure. stdout is flushed
   before writing an error.

scripts/simulate-ednc.py

@@ -74,6 +74,7 @@ TRANSLATION TO THE e10
 """
 
 import argparse
+import os
 import re
 import sys
 
@@ -322,8 +323,37 @@ def main():
         for index, name in enumerate(args.var_names.split(","), start=1):
             VAR_NAMES[index] = name.strip()
 
+    try:
         with open(args.partprogram) as handle:
             fileinfo, general, items = parse_part_program(handle.read())
+    except FileNotFoundError:
+        sys.stdout.flush()
+        sys.stderr.write("no such part program: %s\n" % args.partprogram)
+        here = os.path.join(os.path.dirname(os.path.abspath(__file__)),
+                            "..", "partprograms")
+        for folder in (os.path.dirname(args.partprogram) or ".", here):
+            try:
+                found = sorted(f for f in os.listdir(folder)
+                               if f.lower().endswith(".txt"))
+            except OSError:
+                continue
+            if found:
+                sys.stderr.write("part programs in %s:\n"
+                                 % os.path.normpath(folder))
+                for name in found:
+                    sys.stderr.write("    %s\n" % name)
+                break
+        else:
+            sys.stderr.write("Give the path to a .txt part program. On Windows"
+                             " use backslashes:\n"
+                             "    python scripts\\simulate-ednc.py"
+                             " partprograms\\TEL3L0001.txt ...\n")
+        sys.exit(1)
+    except IsADirectoryError:
+        sys.stdout.flush()
+        sys.stderr.write("%s is a folder, not a part program file\n"
+                         % args.partprogram)
+        sys.exit(1)
 
     print("=" * 72)
     print("PART PROGRAM: %s" % args.partprogram)
@@ -372,7 +402,11 @@ def main():
             else:
                 print("  %-28s [%s]" % (label, payload))
 
-        if args.e10:
+        # `is not None`, NOT a truth test. --e10 passed bare sets this to an
+        # empty string, which is falsy, so a truth test silently turns a
+        # hardware run into a dry run - with no error to explain why nothing
+        # reached the controller.
+        if args.e10 is not None:
             drive_e10(seq, args)
 
 
@@ -412,4 +446,20 @@ def drive_e10(seq, args):
 
 
 if __name__ == "__main__":
+    try:
         main()
+    except KeyboardInterrupt:
+        sys.exit(130)
+    except Exception as err:                       # noqa: BLE001
+        # E10Error is defined in the client module, which is loaded
+        # dynamically, so catch by name rather than importing it up here.
+        if type(err).__name__ in ("E10Error", "E10Fault"):
+            # Flush stdout first. It is block-buffered when piped while stderr
+            # is not, so without this the error appears BEFORE the output it
+            # refers to - and a `| tail` drops it entirely, which looks like
+            # the command failed silently.
+            sys.stdout.flush()
+            sys.stderr.write("e10: %s\n" % err)
+            sys.stderr.flush()
+            sys.exit(1)
+        raise