added better error handling to vault writing, always use a tmp

vault file for writing. added matching unittest to check if
the code catches a disk full error.

tests/unittests.cfg

@@ -329,7 +329,7 @@ dxmorg@florida.cops.gov
 <test check-testkey-invalid-id>
   prepare = ./invalidkeys
   cmd = $pcp -V $vault -S -I testkey-invalid-id
-  expect /invalid key id/
+  expect /(invalid key id|could not decode input)/
 </test>
 
 <test check-testkey-not-encrypted>
@@ -401,6 +401,13 @@ dxmorg@florida.cops.gov
   expect = /ok/
 </test>
 
+# this one only works on freebsd for my user. sorrry
+<test check-vault-disk-full>
+  prepare = sudo mdmfs -s 1M -w 1001:1001 md env && dd if=/dev/zero of=env/b bs=1024 count=700
+  cmd = ./jot 100 | while read N; do if ! (echo a; echo b) | $pcp -V env/v1 -k -x x; then break; fi; done
+  expect = /Failed to copy/
+</test>
+
 #
 # input handling tests
 <test check-large-meta>