Fix KeyError when error happens in library

Fixes vyperlang#252

boa/contracts/vyper/vyper_contract.py

@@ -651,7 +651,11 @@ def debug_frame(self, computation=None):
             # TODO: figure out why fn is None.
             return None
 
-        frame_info = self.compiler_data.function_signatures[fn.name]._ir_info.frame_info
+        signatures = self.compiler_data.function_signatures
+        if fn.name not in signatures:
+            return None  # todo: recurse into imported libraries
+
+        frame_info = signatures[fn.name]._ir_info.frame_info
 
         mem = computation._memory
         frame_detail = FrameDetail(fn.name)

examples/ERC20.vy

@@ -1,3 +1,4 @@
+# pragma version ^0.4.0
 # @dev Implementation of ERC-20 token standard.
 # @author Takayuki Jimba (@yudetamago)
 # https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md

tests/unitary/fixtures/module_contract.vy

@@ -0,0 +1,7 @@
+# pragma version ^0.4.0
+import module_lib
+
+@external
+@view
+def fail():
+    module_lib.throw()

tests/unitary/fixtures/module_lib.vy

@@ -0,0 +1,6 @@
+# pragma version ~=0.4.0
+
+@internal
+@view
+def throw():
+    raise "Error with message"

tests/unitary/test_modules.py

@@ -0,0 +1,11 @@
+from pathlib import Path
+
+import boa
+
+FIXTURES = Path(__file__).parent / "fixtures"
+
+
+def test_throw():
+    c = boa.load(FIXTURES / "module_contract.vy")
+    with boa.reverts("Error with message"):
+        c.fail()