Only write interface to disk if no errors were found

[fwcd]

Fixes #80

The issue was we catch errors from the Curry frontend's sequence of checks here:

curry-language-server/src/Curry/LanguageServer/Compiler.hs

Line 177 in 2a67944

checked <- catchCYIO $ CMD.checkModule opts loaded

...but keep running ExportCheck's expandExports, which triggers the crash:

curry-language-server/src/Curry/LanguageServer/Compiler.hs

Lines 146 to 149 in 2a67944

	-- Parse and check the module
	mdl <- loadAndCheckCurryModule opts m fp
	-- Generate and store an on-disk interface file
	mdl' <- CC.expandExports opts mdl