Print a nicer error message when asked to build an empty string ('').

This happens sometimes, for example, if you do
	whatever | while read x; do
		redo-ifchange "$x"
	done
and the input contains blank lines.

We could ignore the request for blankness, but it seems like that
situation might indicate a more serious bug in your parser, so it's
probably better to just abort with a meaningful error.

builder.py

@@ -282,6 +282,10 @@ def main(targets, shouldbuildfunc):
     seen = {}
     lock = None
     for t in targets:
+        if not t:
+            err('cannot build the empty target ("").\n')
+            retcode[0] = 204
+            break
         assert(state.is_flushed())
         if t in seen:
             continue

redo-ifcreate.py

@@ -9,6 +9,9 @@ try:
                       os.path.join(vars.PWD, vars.TARGET))
     f = state.File(name=me)
     for t in sys.argv[1:]:
+        if not t:
+            err('cannot build the empty target ("").\n')
+            sys.exit(204)
         if os.path.exists(t):
             err('redo-ifcreate: error: %r already exists\n' % t)
             sys.exit(1)

redo-whichdo.py

@@ -12,6 +12,10 @@ if len(sys.argv[1:]) != 1:
     sys.exit(1)
 
 want = sys.argv[1]
+if not want:
+    err('cannot build the empty target ("").\n')
+    sys.exit(204)
+
 abswant = os.path.abspath(want)
 for dodir,dofile,basedir,basename,ext in paths.possible_do_files(abswant):
     dopath = os.path.join('/', dodir, dofile)

state.py

@@ -189,8 +189,7 @@ class File(object):
         row = d.execute(q, l).fetchone()
         if not row:
             if not name:
-                raise Exception('File with id=%r not found and '
+                raise Exception('No file with id=%r name=%r' % (id, name))
-                                'name not given' % id)
             try:
                 _write('insert into Files (name) values (?)', [name])
             except sqlite3.IntegrityError: