Merge InstructionError and ProgramError

From the user's perspective, it's just an instruction error.
For program-specific errors, we still have
InstructionError::CustomError.

programs/failure/src/lib.rs

@@ -1,7 +1,7 @@
 use solana_sdk::account::KeyedAccount;
-use solana_sdk::native_program::ProgramError;
 use solana_sdk::pubkey::Pubkey;
 use solana_sdk::solana_entrypoint;
+use solana_sdk::transaction::InstructionError;
 
 solana_entrypoint!(entrypoint);
 fn entrypoint(
@@ -9,6 +9,6 @@ fn entrypoint(
     _keyed_accounts: &mut [KeyedAccount],
     _data: &[u8],
     _tick_height: u64,
-) -> Result<(), ProgramError> {
-    Err(ProgramError::GenericError)
+) -> Result<(), InstructionError> {
+    Err(InstructionError::GenericError)
 }