avoid a crash when invalid filtering is used and provide a better err…

…or message (#1258)

lib/Echidna/Solidity.hs

@@ -206,6 +206,9 @@ loadSpecified env name cs = do
           Just ne -> Map.singleton mainContract.runtimeCodehash ne
           Nothing -> mempty
 
+  when (Map.null abiMapping) $
+    throwM $ InvalidMethodFilters solConf.methodFilter
+
   -- Set up initial VM, either with chosen contract or Etheno initialization file
   -- need to use snd to add to ABI dict
   initVM <- stToIO $ initialVM solConf.allowFFI

lib/Echidna/Types/Solidity.hs

@@ -49,7 +49,7 @@ instance Show SolException where
     OnlyTests              -> "Only tests and no public functions found in ABI"
     ConstructorArgs s      -> "Constructor arguments are required: " ++ s
     NoCryticCompile        -> "crytic-compile not installed or not found in PATH. To install it, run:\n   pip install crytic-compile"
-    InvalidMethodFilters f -> "Applying " ++ show f ++ " to the methods produces an empty list. Are you filtering the correct functions or fuzzing the correct contract?"
+    InvalidMethodFilters f -> "Applying the filter " ++ show f ++ " to the methods produces an empty list. Are you filtering the correct functions using `filterFunctions` or fuzzing the correct contract?"
     SetUpCallFailed        -> "Calling the setUp() function failed (revert, out-of-gas, sending ether to an non-payable constructor, etc.)"
     DeploymentFailed a t   -> "Deploying the contract " ++ show a ++ " failed (revert, out-of-gas, sending ether to an non-payable constructor, etc.):\n" ++ unpack t
     OutdatedSolcVersion v  -> "Solc version " ++ toString v ++ " detected. Echidna doesn't support versions of solc before " ++ toString minSupportedSolcVersion ++ ". Please use a newer version."