Throw TypeError if passed an offset not supported on the underlying OS (

#84)

* throw TypeError if passed an unsupported offset

Fixes #31

* properly [=throw=]

Co-authored-by: Anne van Kesteren <[email protected]>

index.bs

@@ -1080,6 +1080,8 @@ The <dfn method for=FileSystemSyncAccessHandle>read(|buffer|, {{FileSystemReadWr
 1. Let |readStart| be |options|["{{FileSystemReadWriteOptions/at}}"] if
    |options|["{{FileSystemReadWriteOptions/at}}"] [=map/exists=]; otherwise
    [=this=]'s [=FileSystemSyncAccessHandle/file position cursor=].
+1. If the underlying file system does not support reading from a file offset of
+   |readStart|, [=throw=] a {{TypeError}}.
 1. If |readStart| is larger than |fileSize|:
     1. Set [=this=]'s [=FileSystemSyncAccessHandle/file position cursor=] to |fileSize|.
     1. Return 0.
@@ -1116,6 +1118,8 @@ The <dfn method for=FileSystemSyncAccessHandle>write(|buffer|, {{FileSystemReadW
 1. Let |writePosition| be |options|["{{FileSystemReadWriteOptions/at}}"] if
    |options|["{{FileSystemReadWriteOptions/at}}"] [=map/exists=]; otherwise
    [=this=]'s [=FileSystemSyncAccessHandle/file position cursor=].
+1. If the underlying file system does not support writing to a file offset of
+   |writePosition|, throw a {{TypeError}}.
 1. Let |fileContents| be a copy of [=this=].[=FileSystemSyncAccessHandle/[[file]]=]'s [=file entry/binary data=].
 1. Let |oldSize| be |fileContents|'s [=byte sequence/length=].
 1. Let |bufferSize| be |buffer|'s [=byte length=].
@@ -1168,6 +1172,8 @@ The <dfn method for=FileSystemSyncAccessHandle>truncate(|newSize|)</dfn> method
 1. If [=this=].[=[[state]]=] is "`closed`", throw an {{InvalidStateError}}.
 1. Let |fileContents| be a copy of [=this=].[=FileSystemSyncAccessHandle/[[file]]=]'s [=file entry/binary data=].
 1. 1. Let |oldSize| be the [=byte sequence/length=] of [=this=].[=FileSystemSyncAccessHandle/[[file]]=]'s [=file entry/binary data=].
+1. If the underlying file system does not support setting a file's size to
+   |newSize|, throw a {{TypeError}}.
 1. If |newSize| is larger than |oldSize|:
    1. If |newSize| &minus; |oldSize| exceeds the available [=storage quota=], throw a {{QuotaExceededError}}.
    1. Set [=this=].[=FileSystemSyncAccessHandle/[[file]]=]'s to a [=byte sequence=] formed by concatenating