Adding more error checking for ArcTool, a=chris

diff --git a/src/executables/ArcTool.php b/src/executables/ArcTool.php
index 5d1dd43c2..c81228327 100755
--- a/src/executables/ArcTool.php
+++ b/src/executables/ArcTool.php
@@ -539,7 +539,7 @@ class ArcTool implements CrawlConstants
             exit();
         }
         $shard_count_file = $path . "/reindex_count.txt";
-        if ($start_shard == "continue") {
+        if (trim($start_shard) === "continue") {
             if (file_exists($shard_count_file)) {
                 $start_shard = intval(file_get_contents($shard_count_file));
                 echo "Restarting reindex from $start_shard\n";
@@ -570,8 +570,12 @@ class ArcTool implements CrawlConstants
                     echo "\nShard $i of $num_shards\n";
                     $shard = new IndexShard($shard_name, $i,
                         C\NUM_DOCS_PER_GENERATION, true);
-                    $dictionary->addShardDictionary($shard);
-                    file_put_contents($shard_count_file, $i + 1);
+                    if ($dictionary->addShardDictionary($shard)) {
+                        file_put_contents($shard_count_file, $i + 1);
+                    } else {
+                        echo "Problem adding shard $i";
+                        exit();
+                    }
                 }
                 $max_tier = $dictionary->max_tier;
             }
diff --git a/src/library/IndexDictionary.php b/src/library/IndexDictionary.php
index dcdecea25..8dd8800ec 100644
--- a/src/library/IndexDictionary.php
+++ b/src/library/IndexDictionary.php
@@ -209,10 +209,17 @@ class IndexDictionary implements CrawlConstants
             $out_slot ="B";
         }
         crawlLog("Adding shard data to index dictionary files...");
-        $index_shard->getShardHeader();
+        if (!$index_shard->getShardHeader()) {
+            crawlLog("  Problem reading shard header");
+            return false;
+        }
         $base_offset = IndexShard::HEADER_LENGTH + $index_shard->prefixes_len;
         $prefix_string = $index_shard->getShardSubstring(
             IndexShard::HEADER_LENGTH, $index_shard->prefixes_len, false);
+        if (!$prefix_string) {
+            crawlLog("  Problem reading prefix string");
+            return false;
+        }
         $next_offset = $base_offset;
         $word_item_len = IndexShard:: WORD_KEY_LEN +
                 IndexShard:: WORD_DATA_LEN;
@@ -279,6 +286,7 @@ class IndexDictionary implements CrawlConstants
             }
         }
         crawlLog("...Done Incremental Merging of Index Dictionary Tiers");
+        return true;
     }
     /**
      * Merges for each first letter subdirectory, the $tier pair of files
diff --git a/src/library/IndexShard.php b/src/library/IndexShard.php
index 6e9d0adcf..dcd083247 100644
--- a/src/library/IndexShard.php
+++ b/src/library/IndexShard.php
@@ -1653,16 +1653,21 @@ class IndexShard extends PersistentStructure implements
     /**
      * If not already loaded, reads in from disk the fixed-length'd field
      * variables of this IndexShard ($this->words_len, etc)
+     * @return bool whether was able to read in or not
      */
     public function getShardHeader()
     {
         if (isset($this->num_docs) && $this->num_docs > 0) {
-            return; // if $this->num_docs > 0 assume have read in
+            return true; // if $this->num_docs > 0 assume have read in
         }
         $header = substr($this->readBlockShardAtOffset(0, false),
             0, self::HEADER_LENGTH);
+        if (!$header) {
+            return false;
+        }
         self::headerToShardFields($header, $this);
         $this->doc_info_offset = $this->file_len - $this->docids_len;
+        return true;
     }
     /**
      * Used to store the length of a document as well as the number of